Engine Surge While Stopping

Just got the 340 back from the shop. Had them re-install the intake manifold after I botched the job the first time, after replacing the rocker arms.

On the way home, I noticed that while coming to a stop, (while in neutral, foot on the brake only) the engine revs a good 500 RPM toward the last moments of stopping (highest deceleration), then calms back down to normal idle speed once stopped.

I should note I have MANUAL brakes.

Do I need to hunt a vacuum leak?

Thanks.