The odor changes the response
AC sounds can have more than one explanation. A recording cannot tell us whether refrigerant flow (movement of the fluid that carries heat), a mechanical component, or an electrical problem caused this sound. A burning odor raises a separate concern: Trane notes that an electrical issue can cause a burning smell. Trane AC troubleshooting. How refrigerant works.
If emergency response is needed, get everyone out and stay out until responders clear the situation. U.S. Fire Administration escape guidance.
If there are no signs of active fire, turn cooling off only through a safe, normally accessible control. Do not approach smoking equipment, open electrical covers, reset a tripped breaker, or run another test cycle.
Preserve the repair record
Once safe, record the repair date, work performed, first appearance of the sound, its duration, and when the smell began. Was the sound inside or outside? Did it start during cooling, at shutdown, or when the unit restarted?
Save the invoice, replaced-part information, and any recordings already obtained. Do not recreate the sound for the provider. Tell them clearly that a burning smell followed the recent work so they can assess the urgency.
Ask whether the symptom matches anything observed during the repair and whether an electrical assessment is also needed. The provider should investigate the actual equipment, rather than assure you that hissing is always normal.
Ask for the finding and return-to-service plan
Request the identified cause, any related damage, and the reason for the proposed correction. If the problem is connected to the earlier repair, ask how the provider's workmanship coverage applies. Coverage should be confirmed from the actual terms.
Before reuse, ask how the system will be tested and who will determine that it can return to service. A disappeared smell or a quieter startup alone does not answer the incident.
Your next step is urgent contact with the repair provider, after emergency response if needed, with the invoice and event timeline. Keep the equipment out of service until it has been evaluated.