Pinging/knocking/noise when accelerating and cruise control between 50-70 MPH

I recently changed my oil, transmission fluid, and both coolant systems. I have a 2008 Prius, and I can hear this noise coming from the engine when I accelerate between 50-70 MPH or have the cruise control on.

I had low coolant in the ICE system earlier (didn't know there were two separate systems) and I had the red triangle light up for a moment and "Problem" appear on the LCD screen followed by an orange thermometer. I pulled over and let it cool down, and babied it home and added coolant.

The noise is hard to describe, especially since I don't know what a misfire sounds like. There are no warning lights or error codes, and gas mileage is as good as ever. You can't hear it unless you focus in on it through the background noise, it's very faint.

Anyone have any idea?

 

Just rolled over 76,000 miles. That's seventy six thousand miles. No typo.

 

I changed the coolant because my coolant control valve was bad, and I wasn't aware that there were two coolant systems so I drained the inverter coolant and filled it back up thinking that the coolant system was for both the ICE and inverter. The coolant level has been full for the ICE since, and I have been checking it on a regular basis. It's been the same level, and so has the reserve reservoir. The coolant pump was recently replaced by Toyota.

So what kind of damage could we be potentially looking at? The vehicle operates fine and the gas mileage is unchanged, the only thing is the very faint noise heard between 50-70 MPH. I've noticed on this forum that sometimes people get carried away with themselves and their doom and gloom sometimes turns out to be nothing. Also let me emphasize that the sound is VERY faint and I'm sure that older individuals would have difficulty hearing the sound.

 

I listened to it for a while and it sounded normal. The oil was a normal amber color.

Seems like the sound is starting to go away, I took it up to 55 MPH today for a few seconds and there was no sound. But I'm still paranoid. It's definitely NOT a pinging or knocking sounds, it sounds more like a squeaky shopping cart wheel that's only heard at 55-69 MPH. Performance has not been affected at all. I would like to emphasize that this sound would not likely be heard by older individuals. I'm 22 years old.

I wonder what you think this noise is like? It's NOTHING like what's in this guy's thread: Help, '06 engine making rattling noise | PriusChat

I'm still doing more research, but I'll continue to monitor the vehicle.

 

I did just change my oil, but I've been running 0W-30 for a while. Maybe it's time for a change to 10W-30? I'm in south west Florida. What kind of noise do you hear?

When I put the vehicle into inspection mode, the word "problem" also appears right as the engine starts...

I don't think it does, from what I remember it stays pretty consistent. I'll have to try this next time I'm on the highway.

 

I'm about to go get the codes read at Autozone.

If I damaged the head, I have access to very cheap Prius ICE's. Would it be a matter of replacing the head gasket and head? Or would there also be damage to the block?

I can get a new head for $100, and a head gasket for whatever the price is for a new one online. I would just like to know if replacing the whole engine would be a better option.

Do you think driving the vehicle to the point of imminent failure of the head would damage anything beyond the head, so I can replace it as needed, assuming little drop in performance? I realize this sounds really bad haha

I've worked on pretty advanced weapons sytems on the F-22 Raptor, so I am mechanically inclined with very strict maintenance habits (bagging/labeling ALL bolts, for example). The reason for all this nonsense was that I didn't know that there were two separate cooling systems, and how to properly replace the fluid.

But hopefully this learning lesson won't be too expensive.