vauxhall zafira steering clicking noise?

hi
have already posted a question but could do with some more advice.
have a clicking noise when i turn the steering wheel on my zafira.
universal joint looks fine. had new track rod ends and no play in tyres at all. when it started power steering fluid was low but noise still there since topped it up. steering is fine. not heavy at all.
also the noise isn't there when the car is cold. only comes when its warmed up.
would be grateful for any ideas please?
thanks