Undulating sound from engine

Ariens Sno-Thro Deluxe Series, (921030) snowblower, 254CC engine.
 
The engine, when fired up and running after 1-2 minutes makes an undulating sound.
 
I'll try and get an audio recording but bare with me as I try to phonetically represent the sound:
 
bwwwwWAHwwwwWAHwwwwWAH....and so on.
 
I changed the spark plug last season and just today changed the oil.
 
Any idea what that's about?
 
Otherwise, it works fine.