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.

Does it sound like  rattle or knock or vibration?

Can you think of anything it sounds like?
 

Though it's not an Ariens, it has a 'surge' type of sound like this: https://www.youtube.com/watch?v=VZWpNnXjKAE
The only difference is the thrower in this video sounds like its about to stall out whereas mine just surges and goes back to normal.
I saw another video where apparently most new snow thrower/blowers have a 'hidden fuel filter' in them?

Can you make and post a You Tube video with the engine acting up?
 
Also, if you put the choke on partially, will it smooth out?
 
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