Puzzled by a curious resonance

[Camperfan]

Seeking advice about a curious problem affecting my 1.9-litre watercooled T25 Trakka camper and wondering if anyone can help . . .

The issue is that I'm getting a strange resonance/booming from the engine. It's not happening under acceleration, only when coasting and doesn't appear to be too speed specific (although higher speeds are worse). Even weirder, it seems to go away under braking.
I've been to an exhaust specialist who says there's nothing wrong with the muffler or exhaust but says that if I'd like to spend $600 to replace the entire system, he will, but can't guarantee the problem will go away.
The muffler is an after-market one (slightly noisier than the standard one but I actually lurve the sound under acceleration).
Has anyone else experienced this problem? I'm prepared to replace the exhaust system (it's been welded up a few times but otherwise is okay) if it will fix the issue but wonder if there's a simpler/cheaper solution.
Any ideas??

[gldgti]

maybe its drivetrain related (goes away under brakes).

sounds like a strange problem. could it be an unbalanced cv shaft?