I had the head on my 240 sedan rebuilt about 1800 miles ago due to a small coolant leak. The car had just rolled over to 500K miles. I replaced the new head and for the past 1200 miles, no issues whatsoever. About 500 miles ago, I noticed a very noticeable knock I isolated to the bottom of the oil pan that sounded like a diesel engine does at idle. I hear it with the a/c on but not when the a/c is off. If the a/c is on and I hear the knock and depress the clutch pedal, the knock goes away totally. These are the only 2 things that make the knock go away. Otherwise, all is well with it. Any suggestions on where to begin looking?

Thanks in advance..........