I assume "hard and loud" is when the flame is going and "slow down" is when just the fan is running. This is normal for the flame to come on and off while the furnace is running to prevent overheating the distribution box.
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Ted & Melinda
2016 ACE 27.1
2016 Chevy Sonic Toad
2020 Chevy Colorado Z71 Trail Runner Alternate Toad
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