I Bought a 1996 GMC Sonoma Oct. 2014. that winter when I was not getting heat I had the thermostat replaced which brought on a loud rhythmic clunking sound. Seems to be most noticeable when braking to a stop. Winter passed, the sound went away but with winter back again I had no heat. Had a new thermostat put in yesterday and the clunking is back but I do have heat again. Any thoughts? Mechanic said it could be a head gasket. That sounds expensive.

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    Is the clunking sound always present or is it related to an event like braking or turning? – Zaid Dec 31 '15 at 16:22
  • seems to be when slowing to stop or turn I suppose – Mark Dec 31 '15 at 16:35
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    Does it sound to be related to engine speed, or vehicle speed? You can try downshifting into first or second then stopping, and shifting into neutral then stopping. to determine if it is based on engine speed. Heat issue seems odd to have 2 failures each when it got cold out. Was the temp gauge indicating that the vehicle was not heating up? Has it been loosing coolant? – rpmerf Dec 31 '15 at 18:13
  • I will do this and get back to you. Thank you – Mark Dec 31 '15 at 18:44

Seeing as how changing your thermostat is a variable in this. This sounds more like an HVAC or blower motor issue. If the thermostat fixed it and you now have issues again during winter along with clunking, I would say it's probably a compressor or blower motor issue. Those can cause a clunking sound. Typically you can diagnose a problem with these sounds that are related to your issue:

  • Ticking = Leaves in your blower motor, valves (usually sounds consistent), and Motorized AC control modules (Which are typical for GMC).
  • Clunking = Suspension especially during cold weather, rod knock (it usually sounds very consistent), motor mounts, and sometimes loose stuff under the hood.

I would start by isolating the exact position of this clunking. You'll probably have to have a friend help you. You need to determine if it's under the hood or inside the passenger area. If it was a thermostat, it usually will break a hose or screw up your radiator. It will also overheat. This is because of the extreme pressure due to the thermostat not opening once it's at operating temperature. This could possibly two different issues all together.

A head gasket has two very distinct "give-aways":

  1. it consumes alot of oil or blows blueish smoke.
  2. it blows white smoke, loses coolant and overheats.

Those are the obvious ones. Bad compression, exhaust gas in the coolant, coolant in the oil, and misfires are more in depth things to think about.


Now that you have got heat, I would take a guess that your heating system in now trying to regulate the heat by controlling some of the air control flaps inside the HVAC unit. Maybe one of the motor mechanisms is jammed which is now causing the clucking noise. Do you think the noise is coming from behind the dash?

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