x5 ticking, mayo and sensor?

[JonbmwX5]

hi guys, im have a worrying ticking noice coming from the engine (i think) its there at start up but it can go away after 5-10 mins another times its all the time. its a bmw e53 3.0l petrol and its been fantastic the few years i had it and still love it

the ticking is very load but the car drives fine and has done since its started about a month ago. i had the aa out he hooked it up to the computer and said one of the lamba sersors was showing as a error? could this cause it? he also said it could be a valve flap sticking cos the are plastic? he said he wasnt really sure what was causing it but theses were his best thoughts.

i had it serviced last year and they said the air con belt needed doing, could this cause the ticking? also i found mayo in the oil cap? sign of the head gasket??

i would normally take to bmw specialist but i have only just finished saving up to have it lpg and i know if i take it to the garage i can say goodbye to the system that i really need with the fuel prices at the moment

any thoughts would be much appreciated, thanks jon