[Coco] Buzzing sound in Arkanoid?

[jeremy michea]

Decided to play some coco 3 carts and noticed that Arkanoid seems to 
emit a bit of a background buzzing sound. Never noticed it before and 
seems to happen on more than one coco 3 machine so it doesn't seem to be 
the coco 3's themselves. It's similar to the sound at 24:07 of this video:

https://www.youtube.com/watch?v=--pBWsS867s

Even though it's emulated in the video the sound is similar and only in 
Arkanoid, not in any other coco carts. Is this normal? Like I said I 
don't remember hearing this before when playing it. Not many coco 
arkanoid videos for comparison besides mine (atarileaf) and coco forest 
which is a filmed screen and hard to hear.