My 2001 S-10 4x4 developes a hollow knocking noise that is coming from under the dashboard aproximately in the center of the dash. You can actually feel the thump of the knock up under the dash, but not down on the console or the tunnel directly below? The knock is not consistent with the rpm, in fact does not seem to be affected much by the rpm of the motor and it does not do this when cold. It has to be driven for 1/2 hour to 45 minutes before it developes this inconsistent knocking sound? Is it possible that the heater core could be making this noise? Heater seems to work fine.