Yep, a fast tic-tic-tic is exactly what a puppy heartbeat sounds like.
You are doing very well if you can spot them at 5 weeks; I usually can't find them till 6 or more weeks along. My stethoscope is NOT one of the best, maybe that's why.
Congrats on your upcoming litter! I've got a litter due TODAY, and I think we're right on track.