Noisy tapet sound mainly in 5th gear

Hi came back from a long journey last week and since then poor old landy started making a tappety sounding noise, its mainly more notacable in 5th gear, i have given it a good service since changing every filter, and noticed the rotary filter was still the origional one since 2000, just changed the tension/alternator belt as i was getting a bit of a squeak there, thats gone but still have the tappet sound in 5th, any idea's
thanks:search:

Mmmm crystal ball battery's dying