Good idea to check with speedo disconnected. Mine was making quite a racket and lubing the cable did nothing. All the howling was the speedo - sprayed some WD-40 into the speedo opening (where the cable goes) and everything got nice and quiet.

I don't think my WD-40 fix is the most appropriate way to lube a speedo, but I have a spare I'm reworking and this was a quick fix. About 200 miles ago and still working fine though.