when we start our 2003 Disco in the early morning, it has a high-pitched sound from front, then disappears, does not come back when driving.

is it some kind of belt noise? is there a serpentine belt in this car? our car is 78000km(around 50000mile)

 

Could bea belt tensioner or a pulley, or just a squeaky belt. If you can determine where it is coming from, other than "the front"you can further diagnose exactly what the problem part is.

 

How new are you to your truck? Did it just start? If it's truly a squealing, then as mentioned it could be tensioner or idler pulley possibly. But, when I first got my truck I had it looked at b/c there was a mild "squeal" that then goes away which turned out to be the secondary air pump - normal operation. It's fairly mild, so there's a difference between that and a belt squeal. I'd pop your hood w/ the engine cold, fire it up and see where it's coming from.

 

thanks, there was a shop just done the alternator belt several days ago(before the used car dealer sold car to us), i will check with the shop

 

Find out if they replaced the serpentine belt, also the belt tensioner and idler pulleys all of which can be involved in making your noise.
Mike