[harryinsaudi]

I think/hope it's the belt slipping when the pump is under max load. If the belt looks at all worn get it changed. (Small fine cracks at 90 degrees to the grooves on the inner surface are ok)... but any damage to the outer, or running along the grooves means you need a new one. Of course you may just need the tensioner adjusting. Get the tensioner pulleys checked for wear if you are doing a belt change..... lost one of mine recently.... new rad!