Well, when I went to start Harlem ('00 XJ8 - 180K miles) up after her long hibernation I noticed what sounded like a quick knocking sound from the engine that went away as soon as the engine started. To be sure, I re-started her a couple more times after fully-warmed up and it was still there, and even a bit louder. WTH? I think my car had the same issue before hibernation, but more “softly” in volume/pitch.
I was thinking that it might be a bad spark knock sensor, but if that were the case wouldn’t it continue after the engine started running (and there’s no trouble code being triggered for it)? Same for a rod bearing.
Perhaps it’s low oil pressure … that would explain why it goes away once the engine is running, but don’t know what would cause such low pressure at start-up only to go to needed pressure while running. Maybe bad gas from sitting for many months w/o running (and only a few gallons in the tank)?
Hopefully, this is not the dreaded sound of a loose timing chain b/c of a tensioner about to bite the dust (I don’t know whether this engine had the relevant upgrade or not).
Other than for that starting sound, the engine runs very smoothly and quiet, as it always has. I may try to record the sound on my cellphone and post it as a wav file w. link here, later on.