TBM 940 pilot reported hearing a loud whine noise during cruise climb. During return to departure airport the noise dissipated.

Narrative

On Day 0 at approximately XA00Z; I was operating a part 91 executive transport flight in Aircraft X. I was in the climb phase at approximately FL290 (with a final altitude of FL310); with the autopilot (NAV mode) and auto throttle engaged. A loud whine; which came from forward of my seating position; began suddenly and maintained pitch and intensity. I did not feel or observe any changes in pitch or thrust; and did not detect any changes in performance. I also did not observe any abnormal engine indications or crew alerting system (CAS) messages.I switched the air conditioning system (which has recently been cooling poorly) off; in an attempt to isolate the cause of the whine. No change was observed. I also later cycled the internal separator but did not note a corresponding change.I advised ZZZ center of an abnormal engine noise and requested a return to ZZZ. I also advised there were no other abnormal indications and I did not require priority handling. Air traffic control routed me back to ZZZ; and I requested to maintain higher than normal altitudes until I cleared mountainous terrain northeast of the area.I noticed the pitch of the whine decreased some time after I was established in a descent; and the autothrottle reduced torque. The pitch decrease appeared to correlate directly with a decrease in gas generator speed (Ng) and torque. While in the descent; the whine eventually diminished in volume and eventually went away. I did not observe the whine during the remainder of the flight; including when full Ng and torque were re-established for level flight at lower altitudes. I was vectored to a visual approach for runway XXL and proceeded to an uneventful landing; taxi; and shut down.No quick reference handbook (QRH) procedures are published for abnormal engine noise; and I did not recall mention of emergency procedures (EPs) for this condition during initial training. I reviewed the QRH while descending to double-check my recollection; and found no procedures which address this condition. Due to high ground temperatures (approximately ISA +38-40 degrees C); the takeoff for this flight was conducted in accordance with the Daher high temperature takeoff checklist (which I reviewed prior to departure). The takeoff was conducted with the inertial separator off; autothrottle disengaged; the bleed switch set to off; and torque manually set to 90%. Once established in the climb above 1;000 feet above ground level; I re-engaged the autothrottle and set the bleed switch to auto. I did not turn the inertial separator on; as I was climbing in visual meterological conditions.During taxi prior to departure; I also noted an amber 'OIL TEMP' warning CAS message. I have observed these messages previously when taxiing with high ground temperatures and the inertial separator on. For this flight; the message resolved when the inertial separator was closed for takeoff and takeoff power was set; resulting in improved cooling airflow.I visually inspected the engine bay after shut down and it appeared normal; as it had during pre-flight. The engine oil level in the sight glass also appeared at a similar level (approximately half way into the green band); as it had during the pre-flight.During the abnormal whine event; I took a video clip to document the noise and the panel indications displayed. This clip was forwarded to company maintenance and management personnel.