From what I can tell, chainsaws make a sound like buzzing or rustling, etc. Unfortunately, I haven't been able to find Japanese onomatopoeia from that sound. I wanted to look specifically for the chainsaw sound effect depicted in the manga, Chainsaw Man. Apart from that, it doesn't matter either.
Basically, for very common sounds there is less variance in what onomatopoeia is used, but when it comes to uncommon sounds, there is a lot more diversity.
IMO Chainsaw is a relatively uncommon sound, so it's a bit up for grabs. I'd describe it as something like ブィーン (an established onomatopoeia for electrical tools like drills).