I began to hear thumping sounds coming from the fresh air ventilation duct in my basement. I believe condensation was forming and dropping in the duct. To fix the problem, I loosened the straps or nails holding the wooden joists and ducting, allowing a small amount of freeplay to prevent noise from expansion and contraction due to temperature changes. Learning more about heating, ventilation, and air conditioning systems can help address issues like leaks or noises in the ventilation.