I have just bought a 1999 LR Disco 2. 64k miles, its all good though I have just noticed a noise/vibration which can be heard (gentle growl)and slightly felt under the RHS drivers seat, is this something to do with the suspension? Is it normal? It seems to happen every 20 seconds or so. I previously had a '63 SWB & a 94 disco so therefore never came across this before. Any suggestions greatly appreciated.

 

Sounds like your front drive shaft may need to be inspected for failing u-joints in the double cardon housing that hooks the front shaft to the t/case. Want to crawl under with a flash light and look to see if those 2 u-joints have grease fittings, if they don't get the shaft out and have a shop replace the double cardon u-joints.
One other possibility would be if you have SLS, rear air suspension, your control box and pump or located on the trucks frame under the drivers seat area, you may have a leak and are hearing the pump cycle.
Mike