I have a 1992 deville 4.9 that started knocking about 4 weeks ago. It has 84,000 miles on it. Took it to the Cadillac dealer and had a mechanic listen, he said it is the crankshaft and Cadillac is aware of the problem. Said most start knocking between 30,000 and 50,000 miles and they have replaced entire engines as well as crankshafts and the knocking always returns. He basically told me it won't tear up and I will have to deal with the knocking. How can this be? Has anyone else heard of such a thing??

Leslie

I would like to add that it does not knock when the engine is cold, and it only knocks when it is in gear and it is most noticable when I am taking off from a stop although I can hear it when I am traveling at higher speeds when the engine is warmed up.

Yes, it's main bearing problem, my 1994 Deville with the 4.9 does the same thing at startup with 110k miles. As long as it doesn't get worse, I wouldn't worry about it. There is a redesigned main bearing that can cure the noise, but it's not a required replacement. I may change the one in my car eventually, but I'm not too worried about it now.
No damage is being done to your engine, so the dealer is right, it's ok.

How common is this? It sounds from the dealer that most have this problem. Any guess as to a percentange?

How do I know if it is getting worse? It seems to have gotten much more noticable in the past 2-3 weeks, since I changed the oil.

Leslie,
It is known as "Main Bearing Thump". As Caddydaddy stated it is common, though my last car '92 Deville never had it at 125K. Also as mentioned there is a service bearing that is oversized that can cure it. That said, it is nothing more than annoying and will not cause any problems. You might try using 10W40 oil. That should help quiet it down. Also do a search for "Main Bearing Thump" and read the posts by Bbobynski (a GM engineer). It will eliviate any fears you have.