A faint whine/scream during acceleration can be the fuel injector’s normal sound. I have an older Corolla which doesn’t make that sound, but I had an early VW Rabbit which did. Most people wouldn’t even have notice it in the Rabbit it was so faint. But it was clearly there and most easily heard during rapid acceleration. Sort of halfway between a scream and a buzz I’d say.
I’m sure my Corolla’s injectors make the same sound, just there’s something different about the engine configuration so I can’t hear it. Or my hearing maybe isn’t as good now as then.
If it isn’t normal injector sounds, it could be abnormal injector sounds I guess. But if you have no other symptoms, that’s unlikely. You could try putting some injector cleaner in the gas tank and see if that helps. hmmm … what else? hmmm . well, I presume you’ve already checked all the fluid levels, right? Engine oil, transmission fluid all full & in good shape? Next up, maybe it is the cat heat shield buzzing. That’s a pretty common source of noises we hear here. Easy for a shop to check.
I doubt this is anything too serious. But no harm to ask the seller if they have some ideas what it could be. Probably you’d be better off taking it back to the shop who did the pre-purchase inspection or your regular inde mechanic though, and ask for their opinion. Without actually hearing the sound and how loud it is in relationship to other sounds in the car, hard to tell via the internet.
Before taking to a pro, hyou could do a couple of experiements, Might provide a clue for what the pro should focus on.
- Does it only occur with the engine running, never when the engine is off?
- Only when the car is moving, never when it is parked w/the engine running at idle?
- Only during acceleration?
- Only during ceretain speed ranges?
- If it occurs, then stops b/c you go a little faster, then you coast back through that speed range in neutral, do you still hear it?
- Do the brakes have any effect?
- Any different during cornering vs straight ahead?