Love D90s. It will be my next Rover. Whatever the cause ('normal' wear which should not concern you too much, or something that requires a major intervention) I assume you are going to track it down to ground , given the scarcity and desirability of the vehicle in this country.
Ticking..... 3.9L Rover V8= flat tappet V8 prone to running hot. Thus, it will progressively make more noises with wear, ticking no exception. They also LOVE to get flushed out regularly and run 50 wt oil (here comes the rolling of eyes and deluge of oil comments). In a pinch I used 5w-50 for my air cooled Porsche 911 in my 99 D2 4.0L engine very similar to yours, and to my surprise and delight, it never ran quieter. Same w/ the 4.6L D2 I now drive. I pay the price in lower mileage now and just run 5w-50 synthetic in the 4.6L, even through the winter. I can't say what is going on, I'm no expert on V8s. My experience with aircooled Porsches, VWs, and aircraft is as simple as this: every aluminum and soft metal part in any engine that runs HOT (like pistons, valve rockers, and crank and cam bearings, oil pump housings, to name a few in a Rover V8) all wear over the years and as tolerances grow, thicker viscosity oil is recommended ESPECIALLY those prone to run hot, My own personal experience with 'normal' ticking noises in my 99 D2 4.0 and 04 D2 4.6L is this:
- in both D2s with >120K miles, 50wt oil did in fact produce noticably less 'normal' ticking noises, be they from valve lifters 'sticking' (or being out of spec, as the Rover V8 lacks the ability to individually adjust valve lifter clearance.) or the hundred other oil-bathed parts slapping around under the hood, than 40wt like the Rotella T6. I have done this comparison in both D2s in summer and winter, to satisfy my own curiosity, and I'm sticking with 50wt. I live in MT part of the year so I use a synthetic that is good for cold weather too- Castrol 5w-50, otherwise known as German Castrol in air-cooled Porsche circles since it is still made in Germany. Call me stupid cause it costs a few bucks more. Call me oil ignorant, whatever... that is definitley what has shown to run quietest in my D2s.
- change oil as soon as it starts to look a little dirty, rather than waiting for your favorite mileage/time interval, or once its black. Older engines with cast aluminum pistons (vs forged high output stuff) wear the most and the high tolerance there causes more blow-by combustion gases through the oil. That get's it dirty but also the heat tears down the additives and detergents faster. More miles= more wear= equals more frequent oil changes. We change aircraft engine oil every 50 hrs of operation, which is nothing in an engine that is getting fresh oil pour into it every time its started- the oil comes out pristinely clean, and that's because once its showing signs of looking dirty- it's too late, you've already gunked stuff inside a bit more than you had too.
- I started doing the flush on every change, based on what ive read here from Dmike and others. I use liquimoly again cause the Porsche crowd recommends it for ar 911s, so I have it laying around, but im sure they are basically all the same petroleum-based solvents. My own experience is that once the engine oil starts looking a little dirty I change it, but after 10 minutes of flush, MAN that old oil turns black as tar. Makes you want to flush it twice back-to-back (which I wouldn't hesistate to do if I had a ticking noise that seems to be getting louder).
As regular flushing and 50wt oil cost incrementally nothing compared to the cost of diagnosing and fixing an 'abnormal' ticking (due to the cam chain stretching, worn tappets or cam or valve/guide or water pump bearings, a cracked head or manifold or oil pump gear, or a hundred other 'abnormal' things)-- do that first through two oil change cycles before resorting to anything else, which most certainly will cost A LOT more to diagnose and repair.
I wish you and this fine specimen of the mark the best. Let us know what happens.
PS intersting trivia I came across... If you think the Rover V8 is not prone to high wear or running hot (with weak crank blocks, defective block castings, a stretchy cam chain design, cast pistons, slipping liners, flat tappets without individually adjustable rockers [seriously], frequent head and exhaust manifold cracks, poor coiling), check out chapter 1 in this fascinating read on the Rover V8 (google books will let you read the first 50 pages before making you buy it) isbn:1903706173 - Google Search
. If anything, you will have a new appreciation for ticking noises being the least of your worries