I've got this wierd chirping sound that gives my car a "jolt"

At first I thought I was running over a large dry leaf or something, b/c that's what it sounds like and feels like a wheel just slipped momentarily.

But now it happens like every 30 seconds in stop-and-go driving.

I've been able to reproduce this sound by going into 2nd gear to 3000 rpms or above, then engine breaking. The sound and 'jolt' comes around 2000 rpm decelerating.

I'm absolutely stumbped as to what could be the problem, can anyone help?

Much thanks,