[geezer#99]

That sounds about right on the detonation. When you're driving it and completely release the gas pedal while at 10 mph you might hear it even more. That's due to the higher vacuum when you suddenly close the throttle and you get max vac advance.
Double check on your vac pot when you get back. If you suspect it's shot just hook a vac hose to it and suck on it real hard, put your tongue over the end and then release it. You should hear it snap back. Or you can pop the dizzy cap off and put a vacuum on the hose the same way and watch for the point plate to move.
If you can't figure it out, just post again and we'll troubleshoot it for you.
A good way to pinpoint the source of noises in your motor is use an old broom stick. Put one end behind your ear lobe and put the other end on the block in various places. You'll be surprised what you can hear. Watch out for that fan though.