Another ticking noise

My 1995 Pajero V6 3litre, done over 225 000Ks automatic has over the past few months started to make a ticking noise when the paj trys to pull a high gear in proportion to its speed, I can hear a ticking noise coming for what appears to be under the centre on the car. It does not eminate through the cabin and you can only hear it with a window down. If I put my foot down she picks up a gear and the noise goes away, however if i let it fall back into the same gear the nosie returns if my speed isn't right. The car doesn't appear to be struggling in this high gear, infact it appears to have ample amounts of power to pull it through. Just the ticking noise is bothering me. When I say ticking noise I surpose it is more like a cling.
Any help would be much appreciate.
And thanks to all those who respond to my last posting.