my 2000 disco 2 makes a swishing noise, its when the heater is on, sometimes the flap or something in the dash makes noise too. I figure this can't be normal, but I dont seem to have any leaks and the heat works good. what do I do to fix this, my disco has 118,000 miles and has had its headgasket replaced.

I'm new to these vehicles, what else should I be concerned with? I heard drive shafts (can I just replace the u-joints? are the shaft in the 2004 better, and do they swap?) whats the deal with the head gaskets going bad?, what other mechanical issues should I be looking for? anything proactive that I should be doing?

everything seems to work fine at the present.

 

Go to the DI page and read up in the tech section, most of it is only for DI's but there is some DII info in their too.
The water noise you hear might be air in the cooling system, check the cooling system level.
The type of engine coolant that was used eats gaskets, Dex-Cool, dont use it.
No the '04 drive shaft will not swap, it is a different length.
Yes replace the u-joints and centering ball on the front driveshaft.
Change all fluids and filters, plugs and wires, 91 octane or higher fuel.
Not much else to tell you.

 

Do what Spike says and you'll be good to go