I have a 2004 Disco and when I do a cold start-up, there is a very loud roaring noise from the engine compartment for about 2-3 minutes then it stops. I never hear it again until the next cold start-up.

Also, when I turn on the AC, there is a loud whining noise only during acceleration. When I turn the AC off, I do not hear any whining at all.

Ideas/suggestions?

 

The loud roaring on start up is the lion waking up from its sleep, that's normal. Actually its the secondary air injection pump you hear, this only runs on cold start up for a few minutes. If it's really loud then the pump could be faulty.
The whining noise could be your serp belt slipping, check your serp belt for wear and tension.

 

Thanks! My brother has an 02 Disco and his doesn't make the loud noise during a cols start. Do all Disco II's have the secondary air injection pump?

 

Not all Discos have SAI.

 

Lastly, it only has 42K miles. When does the serpentine belt generally need to be replaced?

 

Check for cracks, its 6 years old and most probably needs to be replaced by now.
This is what you need http://www.roverparts.com/Parts/ERR6898.cfm
Note if you have ACE then you need a different one.

 

It could also be coming from his viscous fan clutch. In that case, that sound is a good thing, means he doesn't have to buy another one this week.........Phil

 

Phil, for which sound? The roar or the whining?

 

roar

Phil

 

On mine the initial noise on cold startup is from the SAI pump, I can't remember the fan ever kicking in. I will keep checking.
Is there a way to test the viscous clutch?