I've just picked up a 159 1.9 diesel with 66k on the clock and have a sound that sounds just like that but it only starts in 3rd gear and then can be heard in 3,4,5 and 6. More revs equals higher pitch, no revs and no sound.
My mechanic said it is probably a gearbox bearing and that it uses a different shaft (apologies if I'm getting this slightly wrong, not mechanically minded!) after first and second gear.
He also said not to worry if not so loud that it is distressing and would cost about 1.5-2k to take out gearbox and fix.
Apart from that I LOVE the car, although not as much as my 156 SW which is petrol and more fun to drive!! The 159 feels more grown up though so they all fill a need!
Depends which bearing, you may have noises on all gears or only on some higher (can't remember exactly what were possible configs).
Regarding repair costs, I paid total around EUR300 (bearings, oil, labour), but it was in Poland, and the bloke who did it is forum member and did repair for me privatelly, so was way cheaper. I know that in Poland AR dealers charged order of EUR500-800.
Mine makes a noise in 5th and 6th - you can change the pitch with the accelerator pedal - almost play a tune if you want. Been like that for many thousands of miles though.
I suspect mine is also connected to the M32 issue, but the 2 specialists who've looked at it both say if it's the usual M32 thing it'll deteriorate rapidly, and certainly within 1,000 miles.
From what I can gather, it's the diesels with the higher torque that are more prone to failing, mine's the 2.2 petrol. Perhaps that's why it's lasting so long, but only guessing.
likely as well bearings. the good news is that you can likely do a lot of miles with the issue - with my 1.9JTDM 150BHP (later upgraded to190BHP) I did around 60k miles/5 years with noisy bearings, and nothing get worse over time (I bought it with the noise). THe only thing I did was I replaced oil (old was with plenty of metal tiny fractions) at start and added some gearbox repair/additive with some ceramics fractions, and then only last year I got gearbox repaired (and the oil was looking good, so degradation probably really slowed)