PMDG 747 -Queen of ther skys (FS9) SOUND PROBLEM

I have just bought the PMDG 747 for FS9 - firstly can i just say what a great product this is. The best aircraft addon by far in my opinion.However I do have a problem. I have a read one or two other people have also experienced this so i know i'm not alone. On quite a few occasions shortly after takeoff I loose all engine sounds. I can hear all the cockpit noises, clicking buttons etc and ATC but no engine sounds. Whatever I do I can't get the sound back and have to reload the whole flight and sometimes even restart fs9. I have also noticed that the sound tends to disappear when you go from virtual cockpit to spot view.It is very irritating to be honest and wondered if there are any pointers??I have loads of adds on e.gGround EnvironmentFlight EnvironmentActive Sky 6 (or latest version)Ultimate Terrain Europe and USALoads of add on airportsUltimate TrafficFreeware watertexturesFreeware tarmac upgradesIn addition to this problem, when the engine sound does work, there seems to be an insessent and loud background engine noise which seems to cover up the main engine sound. It almost sounds like a giant fan and doesn't change volume irrespective of whether the engine is idle or on full power. Its annoying because it disguises the fantastic sound of the 747 engines, which you can barely hear over the background din. When I first installed the 747, it had this background sound, then one day it disappeared out of the blue, and I could hear the engines. I have since has to un-installed and then re-installed the 747 and the background 'fan' noise has returned and I don't know how to turn it off! The sound is there with or without the recent sound update. Any help would be greatly appreciated with both of the above point.Cheers