C30 gearbox/drivetrain harmonic
 

My 2009 C30 DRIVe 1.6 diesel (5 speed) has done 73k miles and has been trouble-free in the 18 months I've owned it. After being laid up for 6 weeks, I took it for a 40-mile run, but after about 18 miles it suddenly developed what I would describe as a 'harmonic' - most noticeable (almost a rumble) in 2nd gear, less noticeable but in a higher 'key' in 3rd gear, very high pitched and much less noticeable in 4th, and not discernable at all in 5th gear (I couldn't pick it olut in 1st gear, but that's pretty noisy anyway).
Does anyone have any experience of this or know what it might be? Thanks.

Inspect the tyres carefully for any uneven wear, especially on the tread blocks - it's a common problem. Our C30 T5 is always having this issue

Failing that, it could be a wheel bearing on its way out.

Lastly, though I would expect other noises such as clicking or "graunching", take a look at the CV joints.

Regards
John