I had a similar issue a couple years ago on my Seedmaster. The rate would always be searching + and - 10%. I lived with it for a couple days but eventually I figured out that when I last changed meter rollers I had not snapped it properly into the bearing holder and it was making it bind and turn hard. So the pwm was constantly opening and closing instead of staying steady.
I don't know exactly how the SH is made but I would check that all your meters are turning easily.