Gearbox oil smells of eggs
Damm I knew it would be some thing....Brush your teeth more often.
Battery over charging?
Its normally a sign of unburnt fuel entering the CAT but if you've not got one its unlikely to be that.
How dies the car run at idle and under partial load... any minor missfires, hesitation or signs of it feeling flat? Have those bodies been balanced yet?
Regarding the cabin filter, pull it and check its not wet.
Only happens when I boot it, like on the private track near donnington, tried to keep up with a mates M3 and it started to smell rotten after the 50mph mark, ive done it several times since down the M1 and it still happens.do you get the smell all the time or just when driving etc ?