Engine noise

[GolfWhiskeyHotel]

Flying a friends Aerostar a few days ago. Really nice ,But the radio......Loud engine (912uls) related noise (change of pitch when increasing revs) Except when I transmit, then all is quiet. No-one complaining of poor reception.  Difficult to hear other's transmissions. Where would I start to look for the problem. Regulator has the large capacitor attached. Radio is ICOM 200. Thanks

G