Strange Vibrations From Engine (Possibly?) While Riding Brakes In Slow Traffic

I tried to search but didn't come up with much that was similar.

Today I had the A/C on and got stuck in traffic because of a draw bridge. Was sitting for about 10 minutes before we got moving again, but it was still very slow because there were 3 merges going on at the same time, so about 2-3 mph max. My lane was slightly downhill so I was lightly riding the brake and I started to feel quite a bit of vibration or rumbling from what I believe was the engine. If I had to describe it, it was almost like a constant and quick start/stop of the gas engine, but perhaps a little rougher. Once I got moving beyond this merge point I didn't feel the vibrations again. The electric motor was at about 2 bars during the time I was feeling this, no warning lights of any kind came on. Anyone experience this before or know what might have caused it to happen?

Thanks.