Annoying sounds from cars at stop light

I would agree Elizabeth. Pretty accurate read on the issue. Sound waves are altered, Some frequencies don't fare well in sheet rock and lose amplitude. Others, like the dreaded 80Hz bass notes, ( those lease busters ) get amplified. 

Alternative explanation: the poor guy upstairs or in the next apartment only hears the bass frequencies but not the rest of the frequencies. That’s why the sound is irritating, it’s just the thump, thump of the bass, not because the bass is amplified. The wall absorbs higher frequencies better than low frequencies.  If it was true walls amplified bass frequencies you could get better bass by constructing a wall in front of your speakers. Hel- loo!

My sound system makes a glorious sound at lights. It's called a Hemi.

I had one of those cars and when I went to the gas station all I heard was a guzzling sound.........

I rented a house in a quiet neighborhood in Honolulu during most of the 70s, and my band built a rehearsing room in the basement by covering the walls with plywood, sheetrock, and mattresses (!). Worked great, and one day some guy from a block or so away comes by after he had trouble napping due to a strange (his words) low frequency sound, and he was relieved to find it was us making that sound. The bass just somehow wormed its way to him, and this was the only time any neighbor had said anything to us about hearing us play...over 7 or so years...