popping noise from driver side floorboard

99 XJ Automatic
stock suspension
190k miles

I have a faint popping/knocking sound that can only be heard from inside the cabin. It seems to be coming from under the driver seat floorboard. You can hear it popping erratically (like popcorn) when sitting still, and it is definitely worse in gear then in neutral or park. I can also hear it when rolling with no foot on brake. It can be felt in the steering wheel and brakes. It's driving me crazy.

As soon as I accelerate it goes away. Car drives great otherwise and is very smooth as long as I have my foot on the gas or am going faster than ~10 mph.

I first checked flexplate to TC bolts, and 2 were loose so I removed, cleaned, and then reinstalled just those 2 with loctite (I know, dumb). The sound seemed to go away for a few weeks then it returned very quickly. I went back under and did the same for the other 2. The sound did not get any better (vibes did). There were no cracks that I could see. Transmission shifts no worse than it has for many years.

I've done motor and trans mounts within past year so doubt it is that. I did replace my exhaust header with an APN, and had to loosen exhaust hangers to make it line up with downpipe/donut. However, hangers are very tight again so don't think it is that either.

I bought a mechanics stethoscope to try and track down the source. The only place I can hear the knock, is by putting the stethoscope directly on the unibody. Then it has a very faint and hollow sound, but I can clearly distinguish it.

I've put it all over the exhaust pipe (testing hangers), engine/head (stuck lifter/tick), motor/trans mounts, transmission/bell housing, belt drive stuff, flexplate area, etc and I can't hear shit. All I hear is a smooth running engine.

Put it on the unibody near driver's side and you can hear it clear as day.

Any ideas as to what the fkkk would be causing this? Other places to stick my stethoscope?

Thanks for the help. This is driving me crazy :confused1

check where the exhaust goes above the trans crossmember. it might be ever so slightly touching.

Check the bellhousing to engine bolts. The dust cover, if you have one, may be loose too on the bell housing. Check for any loose wires that may have connectors as well as transmission cooler lines going to the radiator. My bet is loose bellhousing bolts, though.

Failed transfer case shift lever bushings cause a knocking noise that tends to come and go randomly.

Thanks guys, I will check these out this weekend.

Inner fender

Interested to hear if you found your issue? I've had a similar issue on my '01 XJ and have yet to track down the issue. I've tightened my trackbar mounts, replaced sway-bar bushings, looked for cracks and have yet to find it. Mine may be my brake pads rattling or control arm bushings, or... who knows.

Tim_MN said:

Failed transfer case shift lever bushings cause a knocking noise that tends to come and go randomly.

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I have a XJ that has this. Pulling up on the transfer case shift lever next to the PRNDL makes it go away.

Think all the advice above should get the job done. But if all else fails be sure the TOP TWO bolts on the trans bell housing are tight. On idle (neutral or park) it should be much more prominent than in gear

I was also thinking trans case shift linkage. I usually have to push my lever forward when I come to a stop so it won't rattle.

Redsnake said:

Interested to hear if you found your issue? I've had a similar issue on my '01 XJ and have yet to track down the issue. I've tightened my trackbar mounts, replaced sway-bar bushings, looked for cracks and have yet to find it. Mine may be my brake pads rattling or control arm bushings, or... who knows.

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I'm actually just now checking it out today. I'll report back what I find.

Sway bar bushings most common. Track bar something to inspect considering the danger

ehall said:

Sway bar bushings most common. Track bar something to inspect considering the danger

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I'm thinking it is something along these lines. I'm attaching some pics, apologies for stupid imgur flipping my photos upside down.

The noise was very loud at the front drivers side, under the steering pump. I can hear it very loudly on the front frame where the bumper ties on:



I can hear it and feel it in the steering gear box(?):


Alot of the seals look pretty old and worn, and there looks to be leaks? However, it could be coolant as I had a pretty bad coolant leak (still evidence) not too long ago.



This looks pretty worn out.



There is also a bolt where my bumper ties into frame on driver side that is not connected. Messing with it didn't seem to do anything. I'm thinking I did that when removing the front bumper to replace dampener & radiator. I had a lot of trouble lining all 3 bolts up.


My car feels fine when steering, and I don't get any noticeable noise or grinding related to position or turning of wheel. It happens while the car is just sitting there. If I'm driving it goes away. It will either be an occasional knock every few seconds, or it will go into a fit of 10 or so knocks within a second. Sometimes it will be silent for a minute.

The power steering fluid isn't too low, but it is jet black and smells kinda burned.

It is definitely related to my front steering linkage. I can hear it and feel it all over the trackbar, pitman arm, steering gear, etc. I can also feel many of those wiggling just a tad when I push up and down on it. Given how worn out all of that looks, I guess it needs to be replaced.

Is this dangerous to drive on?

shmicah said:

It is definitely related to my front steering linkage. I can hear it and feel it all over the trackbar, pitman arm, steering gear, etc. I can also feel many of those wiggling just a tad when I push up and down on it. Given how worn out all of that looks, I guess it needs to be replaced.

Is this dangerous to drive on?

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Based on all that it is probably the spacer between the steering box and frame rail. The stock spacer is aluminum and will crack when the inner fender flexes. Lots of aftermarket steel replacements out there.

ehall said:

Based on all that it is probably the spacer between the steering box and frame rail. The stock spacer is aluminum and will crack when the inner fender flexes. Lots of aftermarket steel replacements out there.

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That makes sense. I felt I was able to wiggle the steering box up and down which seemed weird given how snug it is against the rail.

Is it worth upgrading or replacing the steering box while I'm replacing the spacer? What about the rest of the system? Definitely looks like wherever any 2 bars connect needs to have the bushing replaced.

Thanks for your help!

stock box should be fine, id get a new steel spacer and get new bolts cuz the factory ones like to snap, new power steering lines may not hurt and sounds like some new TREs are in order

Reviving an old thread here, but I finally replaced my stock steering box spacer and unfortunately it was not cracked and the issue persists.

I'll be replacing my TREs within the next week, then will start looking into the other recommendations here.

I had a very similar issue until I replaced all my stock control arms with adjustable uppers and lowers I made. No more knocking under hard breaking or potholes.

Tie rod ends usually contribute to steering slop and wobbles not clunks or knocks. Remove your sway bar and report back. Just don't take curves on the freeway at 85. As long as your shocks are in good order you will be fine around town. Personally I've found that dealer bushings at the frame are the best way to go. They are soft and have nylon reinforcement. Harder rubber/plastic from NAPA/MOOG do little to dampen knocks.