It says it has certs:http://solarenergy.advanced-energy.com/en/PVP4800.html
Most of these inverters are two-stage bridge type inverters with Sine Wave output and fully filtered.
It sounds to me like the inverters input or output filtering went bad.
Have the homeowner contact AE and let them know of the interference.
Addendum: Did the installers properly ground Mr. Screamer?
Phil - AC0OB