I had this problem on my 7A. On most take-offs everything was fine and then on one it would be louder than H*** and the first time I thought something was REALLY wrong

When I figured out that the exhaust was hitting the cowl it was a relief.
I couldn't rotate the exhaust in any way to avoid hitting either the engine mount or the cowling. I had lots of people look at it and try to figure out the "right" geometry. This is a cowl for the vertical induction. After fighting with it for months and trying various fixes I gave up and cut the cowl and gave the exhaust pipes more room. I'm REALLY glad I didn't paint it until after all the various issues were resolved. There seems to be a good deal of variation between the fit of cowls to exhausts. Lot's of combos work straight out of the box, many need some minor geometry tweaking and some need surgery! As a first time builder this seemed like a very big deal, but I wonder if multiple offenders simply view this as one of those "cut to fit" things that sometimes are easy and sometimes requires a bit more work.