How to tune an exhaust for noise..?

[notax]

I compete in tarmac rallies in a BMW Compact fitted with a E36 M3 race engine. I run a custom made, twin box exhaust and the system has always run at 97-98db. At the last couple of events the noise level has increased and we have only just scraped by at 100db. The silencers are 'plated' rather than using traditional baffles so shouldn't get louder with use. Anyway, spectators always comment that the car is very quiet compared to rivals' on stage. It seems to be loud at rest at 4500 revs (exactly where it is measured). I know that car manufacturers tune exhausts to be quiet at certain revs to get by noise regulations, any idea how to do this? I'd be quite happy for the car to be noisier but it needs to stay under 100db when tested... I'm also keen to avoid any more restriction in the system.

[notax]

Hmm, interesting thanks. It hadn't occurred to me that resonance could be the issue and that removing restrictions may improve things... Chatting to my mechanic tonight it dawned on me that this problem has only arisen since I fitted a standard engine in place of the full race one which is being rebuilt. It may be that the different cams etc on the race one overcome the issue - although I had thought being more powerful it would exacerbate the problem. In the meantime I need to find a short term fix - do you think fitting a removable 90 degree downward facing bend on the tail pipe to direct the exhaust at the ground would work?