Slight whistle in 4th at low-ish revs

[IntJetSet]

Got a slight whistle in 4th gear at relatively low revs, 1-3k. It's not apparent in any other gear. Anyone got ay experience with this? It's been there since I've had the car but I've only just noticed it (a good indication as to how quiet it is, it's hard to spot with the radio for example)