Notifier System 500 NAC pulse problem

Ok, so I have a Notifier System 509 from 2000 (newer design) that puts out filtered DC. However, when I powered a mechanical horn on the NACS, it made a weird pulse that is coming from the NACS cuz the horn works fine on any other filtered panel. The NAC itself works fine and puts out 24v. When a bell is on the panel, there’s no effect. Any ideas why the NAC is pulsing?