2006 Chevy Trailblazer makes THUD when accelerating

Check your coolant level, low coolant or air in your cooling system will make a "thud"noise in your heater core when the engine is under load from acceleration. The coolant rushes in to the heater core where the air pocket is trapped and then your hear the noise.
Start with the simple and make sure your coolant level is correct and then try to purge the system to remove the air out for your coolant system.
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