Sounds? Or vibrates?
How's compression? Warm engine, remove ALL the plugs at once - block the throttle open, test, spin it so each cylinder gets the same number of "puffs - 3-5."
Record each as you go. Tell us the results.
While running - pull off plug wires one at a time - see if you can determine which cylinder is low power. I know, some folks that don't understand will say it's dangerous to the ignition, not so. Thousands are done that way each day without consequences. Or have someone do an individual cylinder power test with a scope.
If it's not just sound you are hearing it's one cyl producing low power. First you have to determine which one before "fixing it."
A vacuum leak will affect idle more than at higher speeds. A leak doesn't change size - at idle it's a big part of the total volume - at higher speeds it's less of the total.