Where's this noise coming from?

Sounds like spark knock to me. Check/adjust the timing. If you have a dial back timing light handy make sure it doesn't go over 34 degrees total timing with the vacuum advance unplugged and engine revved up to 3000-3500 in neutral. You could also try higher octane fuel if you don't have any tools available. Since you moved to a lower altitude it may also be running a little lean. As 72blu said 2000 ft isn't a big change but if it were borderline to begin with it may be 2-3 jet sizes lean now.