VERY loud sound

'88 MJ 2.5L TBI without A/C.

Had a bad exhaust/intake gasket so I replaced that and went ahead and put in a new exhaust manifold. It was making a very loud noise that we thought was just a major exhaust leak and possible vacuum leaking as the gasket was broken in 2 pieces with part of it around the #4 intake port.

Cranked it up just now and the noise is still there...definitely not an exhaust leak.

Truck runs great...doesn't miss, no high/low idle and it revs fine when you give it gas with no hesitation or stumbles/backfires.

Sounds almost as if it is blowing air or sucking VERY loudly...actually sounds like one of those fireworks you set off on the ground and it spins around until it takes off...but the sound cycle is not quite that fast. When you give it gas and the RPMs climb it all but goes away but as the RPMs come back down it gets noisy again. When you turn it off you can hear it go FFFFF FFFFFFF as if it is puffing a last little bit of air.

Coolant appears to have no oil in it and vice-versa. I would think it it had a head gasket that bad it would not run as well or it would smoke...it does not.

Head gasket... or bad valves/valve seals maybe? don't know much about head work.

But what I can tell you is that that fits the symptoms my M54A2 has to the letter, and the head gasket blew out the back of the head (it's an inline 6 tractor motor! reminds me a lot of jeep motors for some strange reason...) about a week after it started making the noise. I had a rhythmic kind of "fft ffft fffft" noise that followed engine RPMs and became a metallic sounding pinging that also followed engine RPMs at operating speed, it almost sounded like one of those dog training clickers.

I also have the leakdown hissing afterwards.

It ran fine, and I drove it for about a week with this issue before it blew up. Only thing I noticed is that it didn't accelerate quite as well on the highway. It could still maintain cruising speed of 50mph right up until the minute the gasket blew out the back of the head and filled the cab with diesel vapors and smoke.

Here's how you fix unwanted noises...

Could it be a clogged/faulty exhaust system? I would ask if it's quiet when you start it, and gets worse as it warms, but the header leak on my '92 is audible when cold, and disappears when it gets hot, as it is probably swelling shut, as it has done for the eleven years I've owned it. But, a clogged exhaust will ultimately load up and choke out the engine, which you haven't described. The symptoms you've described do sound like a back-pressure issue, though.

Three foot section of vinyl tubing, about 1/2" or less I.D., stick one end in your ear, move the other end around the engine--Standard Warning: clothes and flesh are not a good mix with moving engine components--the vinyl tubing will lead you to the source of the sound.

addition to standard warning... be REAL CAREFUL with where you put that! If you put it too near a source of pressurized air and have it inserted in your ear, severe pain and/or eardrum rupture will occur.

I would put a sheet of pallet-wrap (basically real thick saran wrap) over the end of the hose and ziptie it down. That way you can put it near things and it will pick up sounds nearly as well, but you won't risk blowing an eardrum unless you get very unlucky and stick it directly into an open refrigerant line, a very large head gasket leak, spark plug hole, etc.

We are now fairly certain is is indeed the head gasket. We did a compression check and the noise is coming from the #1 cylinder which only has 60# while #2 is 140+ and 3/4 are at 120 each. So it would seem #1 is bleeding into #2 causing the noise and the lower compression on 1 and higher on 2.

So now to pull the head and then hope the head isn't warped and replace the head gasket.

yay.

Thanks for the input.

It is running again though. So I guess I got a sensor wet or something.