1999 Ford Explorer Engine ticks

My 1999 Ford Explorer makes a ticking noise when the truck is running. I used to own a 96 ford taurus which started to make a similar sound before the heads blew. I took my truck to a few shops to get it checked and each place said there was nothing wrong with it. I've been told by the mechanics to use high mileage oil and that would stop the noise from happening. But it still seems to do it. The ticking goes faster as the accelerator is pressed from 0 to about 30 and noise is worse (louder) while idling and usually calms down when driven at speeds higher than 40 mph. I'm not sure what the problem is but all I know is that my previous vehicle was doing the same thing when it had a problem with the radiator. What should I check or do to find out what is wrong or to fix the problem.