Posting an update - took my detector to a remote beach to test on Sunday and no chatter issues, so looks like my back yard has really bad interference. I was able to solve the issue with the advice in this thread - lowered the sensitivity to 17 and had no issues.
I think the reason i thought it was working previously is i mistook threshold for volume when i unboxed it and was running at a lower setting so didnt notice the noise (rookie mistake).
This video is pretty good at describing the issue - glad i’m not the only one who thought i’d have to return my unit:
