Loud annoying rattle

I have a '96 4.0 AW4 XJ with about 75k miles. I have installed an ARB winch bumper on it, and a 2" RE6160 lift.

Under acceleration there is a rather loud, very consistent, rattling noise, which I can feather with the throttle. Let up a little bit, noise goes away, little more gas, noise comes back.

This has been occurring for about 4 months now. I've looked high and low on the Jeep and cant find anything that would cause this. The noise appears to be coming from the engine bay area. I have secured everything I could possibly think would rattle. The vac bottle, license plate, etc etc. I even thought maybe the motor was shifting forward upon acceleration and the fan was hitting something. No evidence of that either.

Since I can feather the rattling noise with the throttle leads me to believe it may not just be a piece of metal rattling against another piece of metal. If you rev the engine in Park or Neutral the noise does not occur.

Anyone have any ideas of what this could be, or what I should look for? It's very annoying and sometimes it's very loud. Happens with the AC on and off.

I might be misunderstanding the rattling noise you are talking about but are you sure it isnt preignition? Sometimes pinging can sound like rattling. Is it doing it more at low RPMs when you give it gas and it stays at low RPMs without shifting?

One thing comes to mind is a cracked felx plate ? In my experience sounds more like a ticking at first then a louder knock the worst it gets. My chevy was notorious for flex plate cracks, usually dont here anything in park ,just under a load.

Second thing would be a bad fan clutch bearings. Rock the fan front to back, should be no play in that bearing.

If the rattle stops when you push the brakes, it could be the anti rattle spring in the front brake pads. The heat shield for the cat on the wifes 96 would rattle during accelaration. I had a cat on my 88 that would rattle on acceleration, it was something internal that shook loose. When I took the cat off, I could shake it and it would rattle, but on the vehicle it was a sometimes thing.
Check the headlight bezels (or grill), if they are loose they will rattle under acceleration, but it usually goes away when the air pushes on them enough.
Heat shield on the intake manifold and fan shroud also come to mind. Hood springs and about a dozen other things.
I find a rattle in mine and grow two more, I turn up the radio. It will probably eventually fall off and you´ll know for sure what it was. Does drive you nuts though doesn't it?

Atl XJ said:

I might be misunderstanding the rattling noise you are talking about but are you sure it isnt preignition? Sometimes pinging can sound like rattling. Is it doing it more at low RPMs when you give it gas and it stays at low RPMs without shifting?

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Does it usually at higher rpms when I'm "getting on it." Most commonly occurs after a downshift. Doesnt happen, or very little, when I accelerate slowly.

I had a cracked flex plate on my '90, and it made a loud ticking noise at idle. This is a different noise and only happens in motion. fan shroud, grill, and headlight bezels are secure.

I will check the brake springs, and heat shield, thanks.

How do the motor mounts look? I had a motor mount wear on mine and when I would accelerate hard the fan would hit the shroud.

Ryan

On my 2WD 93 the exhaust would rattle where it goes thru the tranny mount under power but not just from reving the motor. Actually got worse after replacing motor and tranny mounts.

Sarge

Maybe this is just a coincidence, but I filled up with a tank of Chevron premium yesterday morning. The rattling noise is nearly gone... Hardly happens at all now, and when it does, it's very faint.

Could the higher octane gas actually have made a difference? What could have caused an interal, low-octane, rattle like that? Or is it more likely that this is just a coincidence, and whatever was rattling shifted so it just doesnt rattle as much?

Yes, It definitely would. That goes back to my preignition question. Higher octane gas usually stops preignition. You probably have A. Need a tune up B. Have clogged fuel injectors or C. Have carbon build up thats causing this.

A & C are probably the most common, but B was what was causing mine.

This happens at higher RPMS and sounds exactly like a rattling noise (not pinging)?

My engine only has 75k on it, and it's very very clean internally. I did a complete tune up about 10,000 ago. Cap, rotor, plugs, wires, a can of GM top end cleaner, fuel filter, etc.

Another thing i have noticed is that the idle is not as smooth as it used to be. Occasionally, the RPMS will go down for a brief second and almost kill the engine (you can hear it when it starts to die).

Clogged fuel injectors maybe? I always run high quality gas (Chevron, Shell, Pacific Pride).

If pre-ignition is my problem, is anything being damaged or seeing extra wear and tear?

Well I filled up with a tank of regular again and the rattling noise does seem to happen more often, and louder, than it did before I refueled after using Premium. However it isn't doing it anywhere near as much as it did before I put in the tank of Premium.

What do you think? Try some good fuel injector cleaner maybe?

Did you solve your problem? I'm interested since my XJ does the same thing.