Disclaimer: I don't even pretend to understand home wiring or how it would affect your gear, nor am I an electrician or electrical engineer. So take this with a huge amount of skepticism...
Could there be a voltage problem when the vacuum cleaner is running? Is it a new vacuum? Or does the vacuum need to have its dust bin, bag, or filters changed? Some vacuums pull a lot of amps from the wall. Many are pretty serious power-consumers. And when they get clogged up with dust, I wonder if they pull even more juice as the motor strains?
Could this perhaps cause a voltage drop at the outlet that is then causing your receiver's internals to produce the popping noise? I have no idea how, but digital things can do strange things when not presented with enough power. Who knows, there could be some D/A converter in your Denon that's really sensitive.
All I know is that I have a Belkin PF-60 glorified surge protector. Arguably, its most useful feature is the digital readout of input voltage and output amperage. Other stuff in my home affects the voltage that my HT system experiences. When I vacuum (yes, I vacuum), and plug into the same circuit in my den that the HT is using, the vacuum does affect the voltage on the PF-60. And before the PF-60, every time my neighbor's HVAC system kicked in (our homes' power runs from the same transformer out above the street), or my wife fired up the clothes dryer, it would send a single, fairly quiet but audible *tick* noise through my HT system. So circuit-based electrical noise can make it all the way to the outputs of a speaker.
