I'd talk to Craig Lincon at carpet cleaners world.
But, I run a 421, same two exhausts, so first is your machine louder than other truckmounts? Why are you getting complaints, are you running it early in the morning or running it at home and your neighbours complaining?
Mostly if I'm on a job and its within normal working hours I don't get any complaints, if I do I just say that I'll be a quick as I can, but I've got a job to do.