I have an '06 M45 and i'm getting some strange noise from the front of the car. I can't tell if its the engine or maybe the front section (catalytic converters) of the exhaust.

Between 2500 and 3000 RPM @ 30 MPH under moderate acceleration I hear what sounds like a dieseling noise coming from under the hood. Its not an octane ping, I run the recommended premium. I know you don't always get the energy advertised in premium fuel but this still isn't that kind of noise.

I first thought it was lifters or maybe a bad valve but that noise would be present all the time not just in specific instances.

I just thought I'd see if someone here has experienced the same thing so I could make some suggestions when I take it in for work.

wow, thanks for the overwhelming responses!

if any were interested the noise turned out to be an exhaust hanger just past the cats. i guess it was hitting something dead cause i wasn't getting a metal to metal 'ping' noise...more along the lines of a dieseling noise.