My only experience being hassled for busking was in NYC near Times Square. The worst I got was cops hussling me along to the next guy's beat until I was well away from tourists (and thus cash).

The First Amendment ought to protect your freedom to busk in the circumstances you've described, but you'll be more likely to busk hassle-free for longer if you wait and talk to Chief Piggenstein. My usual policy is "easier asking forgiveness than permission," but since you've been hassled already, you'll probably face further headaches if you go on. OTOH, I hate asking a cop for permission to do anything I've already got the right to do. Up to you.