Strange noise

My 4L pushrod Ex, makes a noise like high presure air leaking through a blown joint, this happens as the engine gets hot, say if standing in traffic, only at tick over, goes as you rev the engine. The AC leaks a little through the pump body, but filling once a year with a can and gauge its Ok so don't think its that.the belt does not appear to slip. I thought it may be the cooling fan viscous thinging but dowsing it in WD40 makes no difference. Any ideas