Diagnosis by sound, exhaust or something else?

[_M_]

Mine was the opposite. 23 (probably the very first Rs built in 12/22) had the issue at least after 1000 miles but 24 (built two months ago) was day one.

Interesting that it didn’t develop till 1000 miles later. Because my 23 was on day one with the stall start, which means the pump may go bad over time.