Intermittent rap or knock, this occurs during when driving at about 60KMPH and the noise ceases at about 80KMPH, at idle you cant hear the noise at all. the oil pressure gauge moves up half way during drive, but stays close to the lowest during idle, new oil and oil filter has been replaced, but we still get this noise. When the AC is ON the noise stops, but when it is turned off the resurfaces.

Answer: If the noise in your Nissan goes away when the A/C is turned on, then you may have a bad belt or bad A/C compressor. When the compressor engages, it buts tension on the belt because of the extra load. This can cause a belt to stop making noise. Also, when engaged, if there is a problem withing the compressor or the pulley assembly, turning it on can make the noise go away. I would have those checked first.

Next, have all the accessories that are belt driven checked. Any one of them can cause this problem that goes away when belt tension is increased. A bad alternator, water pump, belt tensioner or idler pulley can be the culprit.