Don't forget to inspect the exhaust gaskets, turbo manifold and downpipe. Sometimes an exhaust leak will make that ticking noise. A small leak will be louder when you start the engine and then seemingly go away in a minute or two. A larger leak will be louder then quieter, but still noticeable.

The constant ticking is most likely a result of a mechanical internal component failure of the tappet. This meaning either the valve tappet cylinder check valve or return spring are no longer doing their job. The internal spring is light tension allowing the valve to close at the same time its maintaining the valve lash between the top of the tappet and 45 deg to 90 deg of center hieght of the cam lobe. Unfortunately their are no adjustment that can be made with the hydraulic lifter cam since all the adjustments are made within the internal working of the tappet. Consider trying to replace the noisy tappets.
For more information on Tappets
Visit: http://www.volvoclub.org.uk/tech/HydraulicTappets.pdf

The ticking is the sound of a noisy lifter.
The lifter is not adjusting, most likely, due to air in the plunger.
Air gets sucked into the oil system due to a cracked "O" ring in the oil pan, it is a very common problem.
If your oil level is correct, and the lifter is not damaged (very seldom you find a damaged lifter) the noise may go away on it own.
On my 98 I used to hear the noise once or twice a month and I decided to do a test.
Instead of replacing the "O" ring I made sure the oil level was at the top mark and then I added 1/2 quart extra of oil.
I have not heard the lifter noise in the last 3 years.