Fluttering noise when braking at highway speeds

Hi everyone,

I bought a 2010 Prius with 80K miles on it in 2018. It's my favorite car and drives well, but since around mid-2019 I've noticed a fluttering noise that kicks in when applying more than light force on the brakes at around 50-70 mph (noise increases proportional to speed and how hard I step on the brakes). Starting mid-2020 the noise also sometimes occurs when moving at very high speed (80 mph give or take) without braking.

The sound seems to come from the front on the driver's side; if its minor the passenger struggles to hear it (my wife thought I was crazy at first). It feels like a heavy fan rotating or such, and seems to be unrelated to steering, thankfully. The car is now at 145,000 miles.

Recently I got it checked at the local Toyota dealership, who said they couldn't pinpoint the cause of the noise but did notice worn rear wheel bearings, old spark plugs, and overdue fluid changes, hybrid system maintenance, and throttle cleaning. I had most of the work done elsewhere, then went back to the dealership who did the hybrid maintenance. The fluttering sound seems a bit lessened, but still shows up, especially when braking hard.

Any thoughts on what could be causing the noise/how to get rid of it? I'm willing to put in some DIY work if feasible, but I have a pretty busy day job so I'd probably have to take it to a pro if it gets too involved.

EDIT：
A year ago I got a check engine light which turned out to be a dirty EGR valve, but it went away in the fall. Both the dealership and the other shop said I could ignore it, but from some reading around here it seems that cleaning might be necessary after all？

 

Hi everyone, thanks for the advice. I got the bearings and spark plugs done last year, and recently had the EGR valve cleaned as well. I will have to agree with everyone else who said not to neglect that issue. Cleaning it improved the fuel efficiency significantly, from about 45 to 48 on a fairly hilly drive.

The noise-related issues have gotten somewhat worse as I put more miles on the car (its at 160K, from 145K when I first posted). I'll probably look around the rest of the forum to see if I can keep it in good running condition; my wife thinks we should sell it, but that doesn't sit well with me.

EDIT: The shop that did my EGR cleaning said they checked the engine for leaks and didn't find anything wrong with the head gasket or elsewhere.