Hi Don,

No I'm talking the thud you hear after the flutter induced by too much on the down-line with the ST61. It is a great motor but you need to exercise cautious throttle control. WFO on the up is fine but back off when descending. The beauty of STs is they will tolerate most after market mufflers. The stock unit is large and difficult to fit/hide in the cowl without much Dremel work.

Cheers,

Colin