Helicopter noise on MIDI in C 12 Pro

Every so often, I open Cubase 12 only to find that the sound on all my VST instruments, in every project I open, is distorted by a sound like a “chopper” or helicopter. If I render a track, the sound gets recorded in the audio. Turning everything off and rebooting might work, but definitely not always. I’ve blamed various things (recently added software, cheap plug-ins, etc.) but can’t find a consistent cause. Everything works fine Tuesday evening, but Wednesday morning there’s this problem. I tried looking for answers in the Forum but it always times out, so forgive me if this was asked and answered ages ago. Thanks.