I've noticed it before, and again today in the appalling rain on the M6... when it is really wet or if I've gone through standing water sometimes, there is a really loud noise / vibration from the exhaust when I accelerate. After a minute or two it is back to normal again.
I had the exhaust checked at the last service and nothing appeared wrong.
Could it be that rapid cooling of the pipes is causing them to pull a joint apart or something?
I have HFCs, Y pipe and a scorpion fitted.
Cheers,
Mike