'92 Lumina Vibration/Humming/Rattling stopped in gear

[daszanto]

Mother-in-laws car, '92, Lumina, 3.1 VIN T, 3T40 tranny. If your sitting in the drivers seat, the noise almost sounds like something is hitting the firewall. She says that when she is driving it, and say comes to a stop light, the noise is so annoying she puts it into neutral (noise goes away) till light turns green.

I just finished a major tuneup on it. Car was doing this before and after. I wasn't really trying to get rid of the noise with the tune-up, but just hoping it went away, it hasn't.

Tune-up included: New plugs/wires, PCV valve/hose, tranny pan drop/filter, "dizzy" o-ring replace, engine oil/filter, new plenum gaskets, O2 sensor, SeaFoam into intake and tank, cleaned IAC, new throttle body and EGR gaskets, probably a few more things. I even took off the front bumper and repainted that.

Back to the issue. There where no codes before/after. I've got a MAC Tools TaskMaster which I can view live engine datastream, everything looks OK there.

Thru seaching, I've found possible causes:

1. Checked for vacuum leaks with both carb cleaner and propane, none, except fogot to pull brake booster line and plug it. Could a vacuum leak cause this?

2. One of her dogbones has the rubber bushing split, could it make this noise in gear only?

3. We've got 2 4T60E's that have vacuum lines to them (vacuum modulated?). The 3T40 has no vacuum line, hydraulicly controlled. Is this correct? Auto trannys are my downfall will never understand them.

4. Engine mounts?

I don't have the car now, but want some sugestions of what to check when she brings it over. The quicker I solve this, the less time she spends here, LOL.

Thanks, Dave.

[spiderw31]

It is possible that bad mounts (dogbones included) could allow the engine to rotate enough under load that the exhaust downpipe come into contact with a heat shield or something.

[Lumineer]

Maybe I'm not paying attention to what I'm reading, but regarding the "rattling", have you checked the catalytic converter heat shield or the cat itself?

[ismellrealbad]

I agree with the above. I would get under that thing and look for something thats loose or broken as opposed to worrying about running condition

[Lumineer]

I agree with the above. I would get under that thing and look for something thats loose or broken as opposed to worrying about running condition

Yeah. I remember my Lumina used to have an irritating light rattle. I would start the car and the rattling would start a second or two after running (fast like a vibration), but then would stop after a few seconds. It would also act up at certain RPMS or when accelerating at low RPMs. I got underneath the car to find the rattle (I heard it originating from underneath) and noticed the sound was coming from the cat. I pushed on the heat shield (that cheesegrater-like surface) and noticed it was really loose. By simply messing with it, the thing just fell off (the ridge around the outside that attaches to the flange of the converter itself was rusted to hell). But, the rattling stopped, allowing for the sound of that muscular, rusty exhaust to come through.

[mcbfarm]

I'm with the motor mount theory. Open the hood with it running. See if it moves more at idle than at 3000 rpm. Also, with it off and cool or cold, grab onto the plemum (the coffee shelf on top of the engine) and gently rock forward and back. If you get more than an inch with modest force, replace the mounts. Probably needs it anyway -- few people bother but it's cheap and keeps the exhaust joints tight.