Just picked up 1993 4runner v-6 auto. Expecting to do some work as with any older vehicle. Lots of recent work was done, head gaskets, timing belt, rear end rebuilt and lots more small things...I have spent this weekend identifying the areas of concern but one is giving me fits...it is a noise that seems to come from behind the dash...sounds like valve noise as it follows the motor rpm, but when I lift the hood I cannot hear it...I am due for exhaust next week (cat back) but this is driving me nuts...it changes in volume and sometimes is barely noticeable. Exhaust leak?

thanks