my 960 wagon has a strange pressure release type sound emitting from transmission during down changes from 4 & 3. First noticed after cold start up and encountering load up a hill. This would then go away after car was up to temperature. The noise has recently become more prevalent and persists when the car is at running temperature. On a recent trip towing a small trailer the noise has become more consistent. The transmission otherwise operates normally and shows no signs of slipping or poor performance. I have checked the fluid level, it appears ok and fluid looks clean although it has no been changed for 30000ks. Any ideas would be gratefully received.