I had an aftermarket box on my KV^, which was "boomy" at speed, and also at idle. I did notice, at idle, if I lifted my foot up to 1 of the tailpipes, it went quieter. So a spud was stuffed up it, and a quick drive proved it was quieter, though the spud didn't last long! Sadly this experiment wasn't continued, as circumstances changed and I got the present car instead. Maybe there is something about blocking off one of the tailpipes?
Stewart
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