My 99 Grand Caravan with the 3.8 (97K miles) has developed a very odd noise--a loud grinding or rattle, like rocks in a KitchenAide mixer bowl. It happens only around 800 rpm when the ac compressor comes on. Once the engine speeds up, it's gone. It's usually quiet when the ac isn't running but not always, I can make it happen slightly if I load up the engine. Ocassionally the noise reduces, then comes back.
But it isn't the ac compressor! I located the noise and it appears to be coming from the rear part of the oil pan, from inside the oil pan! Very close to where the engine mates to the transmission. Boy does it get loud.
I'm thinking --ugh!--a rod bearing, but what would the ac cycling have to do with it? Does the load of the ac compressor just sufficiently alter the steady state of the engine and load up the crankshaft just enough at that particular rpm to allow it to fully bang? What would a bad rod bearing sound like anyhow?
Any ideas?
Mark W. in Philadelphia