Stuart,

I am having this issue on occasion, with similar conditions: heat
soaked, i.e. long climb, Mobil 1 syntheitc and get the buzzer just after
letting off of the throttle, i.e. cresting the hill.  I'm also having
very infrequent but associated with the buzzer episodes a split second
valve clatter like once a year, but it will happen two or three times
then not again for a year.

Can you shed any light on the possibility of these events being caused
by a worn pressure relief valve.

Sincerely,

Mark Keller
91 Carat