The intercom had a noticeably irritating alternator whine after installing and testing, pitch increased with RPM. There’s an official noise suppressor from Rugged but is $40 before shipping, import duty and tax. Cheap amazon parts to the rescue, got one for £6 to test out (This one).
Took it to bits to ensure that it wasn’t an empty box (more of a risk on eBay, but also on amazon). There’s parts in there:
Wired this into the power wire on the intercom supply and grounded it to the chassis.
Works great, noise gone!