Rocks in blender sound hours after dealer service

Question: is the situation below likely due to dealership service errors, given service notes further below?

Issue: a few hours after the dealer service I describe below on Friday, I drove my 2011 Prius (155K miles) 1.5 hours with AC constantly on, then parked and continued idling for ~5 mins, keeping AC on. All of a sudden it sounded like there were rocks in a blender under the hood.

Details (skip if too much info): I turned off ignition. Waited a minute, and then restarted still with AC on: immediately the rocks in blender sound returned. I again turned off ignition. Waited a few minutes and restarted still with AC on: sound returned after few minutes. So I called dealership who said they would tow it (at dealership expense) if the sound returned after restarting a couple hours later (when the event I was attending ended) — else ok to drive, but leave it overnight at dealership so they could inspect it (no charge). Fast forward a couple hours: there was no sound after idling 5 mins, including a slow drive around parking lots and side roads for 15 mins, so I drove an uneventful/soundless 1.5h on highway and left car at dealership.

Dealer Response So Far: After inspection the next day (Saturday), the Service Manager told me that technicians are still investigating and need car a couple more days, but looking like head gasket starting to give out. More TBD Monday.

Service Notes (Important To Original Question):

1. Hours before the issue above, the dealership—where I purchased car and have had every service— performed ATF service, oil&lub, and multipoint inspection.
2. When that service above was completed, the Service Manager (SM) gave me records that showed the technician observed the coolant was in poor condition and brake fluid was dirty —so they fixed it free of charge, bc the SM said “I had already paid for it, so you can ignore that.” He was referring to how 7 months/7K miles ago I had same dealership perform recommended Prius 150K “preferred service” slightly early, including coolant drain/replace, brake fluid replace, etc. It seemed odd to me that there were coolant & brake fluid issues bc 7 months ago both were replaced. But the SM said it was addressed in this appointment so didn’t seem a big deal, and I was late for an appointment so I didn’t ask questions. Note: I have the service tech documentation on this.

Specs & Servicing:

1. Model: 2011 Prius
2. Miles: 155K
3. Ownership: I’m the original owner, and purchased new from same dealership where service has always been completed.
4. Service Record: 100% of all manufacturer recommended service (incl. oil changes, xxxK service, etc.) completed on time at this same dealership where I made original purchase, with past 3+ years using same Service Manager.
5. Chosen service packages: usually value, occasionally preferred, once in a while minimum — all based on discussions and recommendations by SM.
6. Car has never had any previous issues with the engine or anything really.

Thanks for suggestions/feedback if this is likely a dealer service issue (e.g., they likely missed coolant and brake replacement 7 months ago), or just as likely an act of nature and coincidence.

 

Thank you both! That helps a lot. I have run the AC a lot recently while idling, bc of a heat wave so it sounds like that might be related.