IF the buzzing were due exclusively to Pianoteq, then the E4 to F4 buzzing would occur even in Pianoteq at extremely low dialed-down volume levels and it would be present on all or nearly all of the selected piano models. Such is usually not the case. In fact, if Pianoteq were responsible for such horrible buzzing, then beta testers or other final users would be screaming to high heaven about Pianoteq being the source of the buzz, long before a given product or version would be released to the public.
In reality, something buzzes because it is an uncontrolled mechanical resonance, either in the enclosure or in the speaker's driver itself. I would also suspect a single faulty speaker would be revealed if you were to swap channels and noted how the buzzing remains in one affected speaker. IF the buzzing remains prominent with one speaker, regardless of which channel it is playing, then one can be reasonably assured the problem is in the transducer. Sometimes a buzzing speaker can be rectified by tightening the fasteners holding the speaker in place in its enclosure. At other times, a speaker driver's foam surround tends to disintegrate with loud playing or simple age.
Cheers,
Joe