My car is a 2003 Cooper S, build date 1/03, with 4,900 miles. I've installed the Rogue Engineering intake about 3,000 miles ago.

I have what is best described as a hiss coming from the engine. It rises and falls with revs with no correlation to clutch engagement or whether the car's in gear or not. The hiss is clearly audible at idle.

This is not the supercharger whine, nor is it induction noise due to the Rogue Engineering intake. The hiss has only recently developed.

I've looked under the bonnet/hood and it sounds like it's eminating in the middle of the car at the fire wall. It is not coming from the intercooler, at least not from the fittings on top of the engine.

It feels like I've lost power, but only marginally. I could be wrong on this and my perception could be only that, my perception.

Does anybody have any idea what this could be?

I will put the car up on stands today to see if I can further localize the sound.

If I can diagnose it and fix it myself I'd prefer to. Rather I don't want to re-install the stock airbox and deal with the less than co-operative local service dept and take time off work to do so. Help me out and I'll send you a case of your favorite barley pop.