Hello.

Since 2 new Polycoms are having the same problem, it is unlikely (?) that the Polycom is the problem, unless you are very unlucky or there is a bad design.

Since you verified that the line is not the problem, check its physical location. See if there is any interference from lights, sounds, including projectors. Move the Polycom to a different room to see if the same problem occurs.

Hope this helps.

Has it always been like that or it just started. this can be a speaker gone bad or an unstable line. a simple service may correct that effectively.