Is this noise suspension or engine?

[ChuChi]

We have an '87 GL wagon. EA-82 5sp-dr.

For the past few weeks my gf has been complaining about a "banging noise" that the car would make while driving up hills. Actually, just one hill in particular that's fairly long and driven at freeway speeds (26 over sylvan hill for you Portlanders) however it doesn't happen every time. This is the only place where this noise has ever occurred and once cresting the top and starting the coast downhill, the noise goes away. We've driven this strech of road many times in this vehicle and others without incident so I'm ruling out the road surface. I'm thinking that it's the ~2 mile, high speed, uphill that brings this out. I've tweaked with a few bits (spark plugs, disti, lugnuts, carb, vaccum leaks) in shot-in-the-dark attempts to affect it without being able to experience it for myself.

 

Tonight it finally happened while I was in the car. She was driving. I'd describe is as more of a 'thumping' than a 'banging' noise. Like the deep base of an exhust leak, something heavy hitting tight sheet metal, or the pounding of a drum. We can feel the vibration in the car. It's slight (my gf started 'ahhhhhhh'ing to make her voice warble) and not upsetting the handling, but strong enough that it feels like something could be impacting and concerning enough to kick in my "pull over" reflex. It's definitely speed, not RPM, related.

 

Tests we did to try to narrow it down:

Swerving in the lane with quick jerks - no change to noise/vibration

Swerving in the lane with long, slight turns - pitch of noise raise slightly and intensity seems to decrease, but no difference from left to right

No throttle, clutch in - noise/vibration stops

1/2 throttle, clutch in - noise/vibration stops

No throttle, clutch engaged - noise/vibration stops

Holding speed and shifting gears (varying RPM)- no change

Downshifting and holding RPM (varying speed) - the frequency of the noise changes

 

So it only happens when the engine in under load, which leads me to the engine. But it's only affected by changes in speed or handling, which leads me to the driveline, axels, wheels, etc. I'm stumped. Any ideas?

[4x4_Welder]

Pull the rear driveshaft and check your universals, I bet the front one is seized. You may also have a bad motor or transmission mount, or torque arm.

[ChuChi]

Ahh, mounts. That seems too obvious. Will check those this weekend.

 

For the u-joint and torque arm I don't think I understand. This is happening while the car is in 2wd. Isn't everything rear of the transmission free spinning?

 

 

(Also, thanks to the mod who moved this to the proper forum.)

[4x4_Welder]

Yes, everything is free spinning, however being in FWD will cause the whole engine and transmission to try to rotate as more power is being applied. This is the purpose of the torque arm from the bell housing to the spare tire mount, and if this doesn't control it, or the motor/transmission mounts are soft allowing movement, it can change the angle the forward-most universal joint sees. These typically seize due to being non-greasable, and just generally being ignored.

Next time you drive the car, try a little bit of hard acceleration in first gear. If the problem is what I mentioned, then you should feel and hear the noise at a much lower frequency.

[zyewdall]

Another thing to test. Throw it in 4wd and see if it reduces the vibration. I had an odd similar (speed sensitive, but only under hard throttle) vibration in mine that mostly went away if I put in 4wd, and it turned out to be a front CV/axle going bad. I guess in 4wd it has less torque on the bad axle, and it didn't vibrate, but in 2wd, all the torque was on the front axles and it showed up.

 

Z