I agree. This shouldn't be 'normal', and it might actually be a conscious decision on the part of BMW to reduce the quality control and 'cash-in' on the marque.
I can't say I agree. My parents used to have a Z4M and whenever I borrowed it, my pals would always want to hear the engine with the hood up. We were constantly comparing the sound of that with the sound of the Corvette engine a guy round the corner had. In the end, the consensus was that the Corvette engine had a better volume, but that the German-engineered BMW had a much better note and 'tightness' about the sound.
Believe me, the number of times we were meticulously compared BMW vs. Vette (the group was pretty much split down the middle as to which engine everyone preferred), we would have noticed an odd sound like that!
I don't know if it's a hypochondriac syndrome, but it is strange that nobody noticed it on their test drives. But then, sometimes it's difficult to detect a problem in one place, when a nice feature somewhere else distracts you. lol
