2003 Volvo S40 engine rattles

Is the sound a rattle or a tick? If a rattle, have you checked the heat shields on the exhaust? Have you checked the catalytic converter. I have seen them come loose internally.

No loose heat sheilds and cat ok, definitely engine internal, top end I'd say

If it is a tapping, chances are it is a lifter.

Yes it's a tapping sound noticeable more when engine is at it's normal operating temperature and when free revved above 3000rpm.

You could try using an additive such as Lucas, BG, STP... It may help.

Ok will try that but I'm thinking it could be more serious an issue as last evening the car overheated quite badly, water temperature gauge was reading normal, fan was kicking in as normal and plenty oil and water in the engine, removed oil filler cap and all seems ok, no smoke or smell of burning oil but lost all power and had to crawl the last few miles at 15mph.

Have you confirmed that the catalytic converter isn't plugged? Does it get red hot when the engine is running for some time? Also, since you lost all that power, did the check engine light come on?

PLEASE NOTE: Since this new issue with power loss, I agree with you that an additive most likely isn't going to do anything. Let me know the answers to the questions above.

Have had the car on the ramp and checked the cat and all is good, when it overheated no warning lights came up at all. Have since driven the car and the overheating seems to have lessened (boiled over twice more when releasing the radiator cap but now just steam vapour).

Are you sure the rad cap is good? If you only see vapor, are you sure it isn't low on coolant again? Also, are you using coolant or water to fill the system?

Rad cap is fine and I'm using coolant, hasn't overheated again since the boil overs, thinking it could have been an air lock in the system somewhere, rattle is still present though with no change, as you said it maybe a lifter, is there a way i can check this without major engine surgery?

The only thing I can suggest is to remove the valve cover and see if anything appears loose.

Ok will have a look.