I can hear a low rumbling under the hood and took a look. I think the idler pully for the one-does-all fan belt is causing the problem. It is bouncing a lot. Rumbling is worse with the increased load of the A/C compressor.

Did a quick search - know this has been discussed before - but can't find it.

So, I'm now looking for tips on belt removal and idler arm pully replacement.

Records show it was replaced about 50K miles ago. Apparently not a long life thing.

It could be another device, but this pully would be the first suspect. It is running straight, not sideways, so maybe not critical at this point.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles