The engine of my car is going to be swapped with a newer one and I recall having issues with my clutch engagement where sometimes it makes a chattering sound. I tried to reproduce it this evening, but oddly I haven't experienced it in awhile. My mechanic suggested that if there is an issue with the clutch pressure plate then it will be more noticeable when hot.

I only went to the grocery store and back this evening, so only about 12 miles round trip. I tried to reproduce the chatter, but I could only reproduce it going up a slight hill and keeping the RPMs down:

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Just replace the clutch with the engine. Could be clutch wear or oil contamination.