I'm with you on that one Brocket. We were sitting in the car in traffic, on a dual carriageway, a few weeks ago and I even had the drivers window down.
I spotted lights in the wing mirror coming up the white line. I made out at least two bikes filtering, 5 maybe 6 cars back. I never heard anything until they were next to the car behind me & then not a great deal, but as each of them past the noise was 'quite loud'. There were 4 of them all 'Harley style' bikes with open pipes.
As they continued I could hear the last one, probably 4 or 5 cars away.
This I've noticed before, you can't hear anything coming from behind as well as you can hear it in front of you.
Yes, I'm an old deaf t**t but so are a lot of car drivers, ride defensively, not aggressively & don't expect others to see or hear you.