Rudd Furnace Strange Noise

[Adman21]

I have a 1993 Rudd High Efficiency furnace.  I have always done regular maintenance on it and have replaced a few parts over the years.  When it's warmed up and blowing heat there is an audible sort of metallic sound that can be heard from at least 20 feet away.  Sounds like something that has worn down and is on the verge of going out.  Any ideas what could be producing this sound?

[JWWebster]

Might just be loose duct work? Maybe a few self tappers would fix that.