smithe213

Hey all,
Last fall I started getting a noise that sounds like a resonant vibration in my truck. I first noticed it after doing something to it-I don't remember now if it was a front diff swap (to diff w/ARB) or clutch replacement but it was one or the other. The noise is loud and annoying in the cab- It sounds as though the engine or trans is "grounding out" or touching the body someplace and transferring vibrations. I would call the noise a hum or a buzz more so than a rattle- it doesn't have very much of a "tin can" type sound, it sounds tighter. The sound is only present above 1700 rpm when it comes on rather suddenly. The noise occurs when the truck is stationary or moving, at any grade, level uphill or downhill and under any amount of load, it seems to only be dependant on RPM. I can't hear the annoying noise anywhere under the hood but from tying a strap to various things I was able to try to narrow it down. The sound appears to originate in the center of the firewall/ passenger side firewall or at the front of the transmission tunnel but it is hard to tell for sure where it's coming from.

Sounds to me like two things touch that shouldn't so I checked the following while at 1800 rpm with the noise present and the noise persisted:
-Pushed on the dash everywhere and in every direction possible. No change in rattle
-Opened both doors, lifted and pushed down on doors. No change in rattle
-Removed glove box and pushed and pried in serval locations on the heater box. No change in rattle
-Pushed/pulled/pried steering shaft and steering column in several places. No change in rattle but realized steering shaft seal to firewall is bad.
-Removed both kick panels. No change in rattle
-Looked under the hood and under the truck for anything at all touching from engine or trans to body/chassis. Removed rock jammed in transfer case mount. No change in rattle.
-Checked exhaust. Kicked in various places and nothing made noise or was loose. Upper bolt in bracket that bolts to bellhousing and that that down pipe in clamped to is missing the upper bolt (on purpose). When clamped with vice grips no change in rattle
-Wiring harness near intake plenum touches firewall. When pulled away no change in rattle
-When heater hoses to core or AC lines to evaporator are gently moved, vibration persists
-Brake line clamps on firewall are all tight-I didn't look closely down by the starter
-Screws are tight on PAIR valve

Truck is a 1994 Regular Cab, 22re, 5 speed 4x4.1" BL, ARB compressor under hood and front/rear ARB lockers, BJ spacers, rear AALs, homemade bumpers and front skid plate, otherwise mostly stock if it makes a difference.

Any help or common spots I've missed would be appreciated. Sorry for the long winded post.

scope103

Sounds like a PAIR valve, but you say you've checked it. Checking it is as easy as putting your finger on the cover plate; if the noise stops, that's it.

Other than that, I'm no help.

smithe213

Thanks for the suggestion. Just to be sure tonight I double checked. All screws are tight, finger on the PAIR still has vibration. Checked as per the FSM and the valve is working correctly as well. I noticed tonight where the coolant tube that turns up and the heater hose slides onto at the left rear of the engine there's something sooty on the engine side of the tube. Almost seems like the EGR crossover plate on the back of the head is leaking. Has anyone had this happen? The two bolts I could see looked tight, and I didn't feel it leaking when I stuck my hand behind the engine but I'm not sure I even would, EGR may have been closed. Is there anyway that gasket would have somehow blown out?