If you think the noise is coming from inside the sump, and as IIRC your car is the 3500S, then as you don't need to remove the exhaust to drop the sump, I'd suggest that for the price of a sump gasket you drop the sump and have a look inside. You might see a witness mark or see something else that might be causing it. You could also wait until you get the noise, then pull the plug leads off one by one to see if the reduction of load on the piston reduces the noise.