100 series 1HZ ticking noise

Hi everyone.

I purchased a 2002 100 series about a week ago.
Problem is the engine is ticking once it gets up to operating temp, 1200 rpm onwards mainly when engine has no load.
Seems to be quite loud underneith but you can here it up top to, but makes it hard with all the other noise in engine bay.
Toyota is having a look at it tommorrow for me i just thought it wouldnt hurt to get some ideas of you guys, as they seem to think it is gear box wich i disagree with.
Only ticks once it is warm the pressure gauge sits on half once warm and nearly 3 quarters when cold thought this might have something to do with it. Definately engine noise, can only notice noise with window down i test drove with windows up (stupid), I have read maybe injector's or valves. Please help.