An update, I visited a wet, but quiet Bedford Autodrome today, with a freshly packed silencer ( Acousti-fill) and the Caterham track day airbox. Bedford measured everyone at 4,500 rpm, I was only 93.5dB so no problem there. I then did a dozen laps building up revs/ speed and eventually I did get one noise warning, which they give you if you are within a couple of dB of the drivebys After the warning I kept below 6,000rpm only by the noise meters and there was no problems. For the last few laps, before I was going home, I did my best to get black flagged for noise, to see if it would happen, it didn’t... I think a little more noise reduction work to do, but then i should be safe on any track. Also worth noting I had the car re-mapped to suit the airbox over the foam pipercross it came with, I also had to use 30mm shorter trumpets, we ran the car with/ without the airbox and the power was down by 15BHP ish using the airbox, but the noise difference on the rollers was huge!!