Collapsed Lifter or bent pushrod. Pull the plug wires off one at a time until you find the one that changes the sound. I had this happen to me last year on a 87 318 in a van while pulling a hill. I wasn't stomping on it, just driving it. It did exactly as you described. The lifter collapsed and the pushrod was stuck in the lifter. The motor only had 80,000 miles, 1 owner (me) and never abused.