Strange engine noise

[MattMW]

Hey guys, need some help on a diagnosis.
My 2005 wagon has been making a unusual noise lately. It started a few weeks ago when I done a coolant flush which resulted in a split hose and over heated twice a few weeks apart.
So pretty much since the over heating it's been making a deep "burrrr" sound when accelerating or going up a hill. It doesn't happen all the time and I can't notice any pattern of when it happens.
I'm suspecting some expensive internal damage

 

[carcollector]

Maybe you just got a crapload of coolant on the serpentine belt, tensioner, alternator, A/C compressor etc. How badly did it overheat, and for how long? Does it still run at a normal temperature? Does it still have the same power and run smoothly? Is the oil still clean? Does it still show correct charging voltage when running? Any error codes?
Try turning the A/C off when you hear the noise, and see if the noise changes or goes away. If it does, it's probably not serious. Turning off the A/C reduces the strain on the serpentine belt. It could just need a good cleanup and a new belt, or possibly have an issue with an alternator or A/C bearing rather than something wrong internally in the engine.

 

[MattMW]

Hey so it over heated twice and both times for no longer than 2 minutes, I put a new belt on about 3-4 months ago and it's pretty clean with all bearings in tact. After a few days of driving I've noticed that it only makes a noise at 80km/h+ and when I put my foot down it goes from 1600rpm up to 2000-2500rpm but drops straight back to 1600 and that's when the "burrrr" noise happens. It happens independently of AC/radio/lights or any other electronics. I've started thinking it's a A:F ratio problem. My instantaneous fuel goes from around 6L/H to 15L/H back down to 6 again. Could this be a hardware or software issue?

 

[MattMW]

Also all filters/fluids and spark plugs have been changed recently and since I've owned it I've put fair amount of fuel/oil cleaners through and serviced it thoroughly and regularly