Knocking sound

I've got a 2000 chevy tracker and i've recently started to hear knocking sounds coming from under the hood, it only happens while going uphill, then starts to struggle at about 50-70mph, the sound gets louder when the a/c in on.It has the correct level of engine oil and transmission fluid. Although due for an oil change, what could possibly be causing the knocking?

if its a knock you hear it could be your crank

if you hear a clack its your valves

but if you hear a ping then use a higher octane fuel
pinging occurs when you put the engine under load while using crappy fuel