Knocking noise

My 99 Toyota Tacoma runs fine until it gets warm
and under a load (pulling a trailer). Then it begins
to rattle like valves rattling but when you let off the
accelerator it goes away.   Also when in town at a
red light it is silent also.  I drive this truck daily and
it never makes the noise until I hook it to the trailer
and then it only does it once it warms up on a hot
day (after about a hour of driving).  Any advice
would be greatly appreciated. Thanks in advance