Oversensitive (or any) car alarms. Pointless too as everybody seems to ignore them. My former brother-in-law, many years ago when living in Hatfield, heard a car alarm going off in the middle of the night. He ignored it (well, it was Hatfield, and car alarms may go off more frequently and for good reason there unlike may other instances). In the morning, he discovered it was his car and that it had indeed been broken into and all contents taken (with damage caused).
Closely followed by the key fob button to confirm your car is locked by making the horn beep (although I read that these are banned in Europe?). So unnecessary and so noisy, and people seem to have a habit of doing it from several meters away while I am next to their car. At least Hondas emit a reasonably unobtrusive digital beep.
Also reversing alarms (some people are fitting these to their cars too), and noisy exhausts.
Noise really; and not a silly little thing either. I can half remember a quote about "every little bang" sending somebody off their trolley, but can't find it now.