The 904 in my '65 V-200 has a rather annoying problem (or perhaps multiple problems). Perhaps someone can offer some advice.

When the engine is running, the shift selector is in D, 1, or 2, and the brake is applied holding the car at a stop, there is a somewhat loud, rhythmic clunking noise, a low growl, and a good deal of vibration. Upon accelerating, the car shudders and growls a bit, and then seems to be alright when it gets up to 10 or 20 MPH. In P, N, or R, there is no problem when the car is stopped.

What might this be?

Have you checked the muffler bearing?

Oh, sorry.

I couldn't resist.

Seriously, car in gear and wheels not turning... I'd be looking at the front pump in the transmission and the drive hub on the torque converter. Those hubs have been known to break every now and then.

I have the clunking noise under my car. Checked underneath and it turned out the muffler bearing was frozen. In order to check this one must be in an un-ventilated cramped area with the car running and staring straight down the exhaust pipe. Please, please don't tell the medics you read this post when they inevitably show up...

How are your motor and transmission mounts? If they've gone soft, they could be allowing the engine/trans to hit something. The engine moves a bit while in gear with the brake on, and might be making contact. Being in reverse, or driving could change the position enough to eliminate the noise. Check the mounts, K-frame to pan clearance, etc.

The guy that had the car before me somehow managed to install a muffler bearing from an old Toyota, and the bearing housing can't be opened without a left-handed metric crescent wrench, which I have special ordered from a local tool store. I'll let you know what I find out upon inspection

But seriously...

How will I know if the mounts are still good? How do I know if they are hard enough? Is there a good way to check this out short of laying under the car while someone's got it running and in gear?

Yes, using caution , lay in front of the car looking at the motor (don't need to lay underneith it.) Your friend (don't piss him off) can start the car and run through the different gears R, N, D. If the pan is hitting, you'll be able to "see" that it is the origin of the noise. If the motor really jumps around, the mounts are bad. If the rubber is torn or smashed or just looking really old, just replace them.

Or from the top, put the car in drive with foot on brake lightly rev the motor. It should move, but not flop around in there. Mounts arn't expensive to replace.

I'd also take a good look at my exhaust pipes and tailpipe, since they are common clunkers.

What is your idle speed? Too fast of an idle will make the transmission engage a little and rattle everything at a stop. My cars make a grinding noise just as the transmission engages which is not problematic. If the transmission is almost engaging at idle, this noise would be annoying and sound like a big problem that it isn't.

Have you checked for vacuum leaks? They make engines run rough.

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1980 Aspen 225 super six

when you get the new exhaust bearing be sure to lube it properly, we cant stress this enough. my personal recomendation is synthetic lube, but it is expensive.

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