I have Subaru Impraza 07. In last couple of weeks I am hearing metallic popping sound when I hit accelerator to 2000+ RPM. I don't hear this noise if the engine is cold but getting heated this sound starts appearing. I would be grateful if anyone have answer.

could be the waterpump bearing you hear as you accelerate.

check you brake pads, there might be no pad left so you stopping metal to metal if this is the case you will need brake pads and rotors

will try to assist you. did you have your clutch replaced professionally? if it's a popping or cracking sound when accelerating or turning, it's most likely a c.v. axle going bad. either on driver or passenger side in the front of the car right?if it is a bad c.v. axle it will pop and crack when you turn wheel from left to right at low r.p.m. if not this,you may have brake issues. either it's a grinding sound while accelerating or decellerating,as long as cars in motion the noise will persist.try tapping your brakes while in a parking lot in slow gear,hear something from driver front,have a passenger with the widow down on passenger side, hear it from front passenger side? the thing i go by is what i hear when i test drive to diagnose an issue with a vehicle is,axles?popping and cracking when turning the steering wheel from left to right at low speed. [axle problem] grinding,scraping sound? brake issues! hope this helps and thank you for choosing fixya.com and we are here for you 24/7 7days per week thank you!!!!

Sounds like an Exhaust Manifold leak on only one cylinder. The Gasket is probably blown out. Check around where manifold is bolted to the engine. Any leaks are usually evident by a black sooty deposit.

It is very hard to give you a reliable answer without knowing or hearing the noise for myself. I would take it to another local shop for a real mechanic to listen to the noise. It really ticks me off that the parts changers you have dealt with are so Un-helpful. When working on a Subaru they should have more respect for the customer. Anyway, I would take it to a shop in your area to allow another person to hear the noise. If you give me your area, I will suggest a shop in your area that maybe better at diagnosing your problem.

Sounds like your CV half shaft is has gone bad. More than likely when you look behind your front wheels you will see a rip or hole in the CV boot. You will need to replace the CV joint to fix this. Good luck and thanks for using FixYa!

Alternator?
Voltage Regulator?

If the noise sounds like a weeeeeee that increases in pitch as the RPMs increase, and you haven't heard this sound before the 'problem', suspect the alternator...

it is big end bearing.

Try rear wheel bearings. The cause a roaring noise that increases with acceleration. Sounds very similar to loud road noise.