Could If it's smooth while it's flying, I'd just fly it and not worry about it. No sense in making it super perfect if you're just going to make an error and crash it anyways. j/k

Seriously though, it's probably just the dampers (those rubber o-rings and washers) centering themselve during spool up. My brushless cpp with hard damper settings still vibrates a little during spool up, even when it's perfectly setup.

kev

[edit: Since your chopper is used, it might need some lube on those dampers. I use 10,000wt silicone diff oil meant for 1/8 scale nitro buggies. Silicone R/C shock absorber oil also comes to mind. I prefer the diff oil because it's very viscous so it won't get "spun off" as quickly, but really any silicone oil will get the job done and allow your rotor to center itself earlier during spool up.]