hearing a knock when engine is revved upto 2500 rpms

That's never good. Rod knock? Does the knock go away above 2.5K?

Knocking isn't a good thing. How fast is the knock, and does it get faster as RPMs increase? If the knock is more of a "tick" that's really fast, it's likely in the valve train. If it's slower and more "low pitched", it's probably rod knock.

It could also be piston slap on cold starts. Does it go away once the car warms up?

youve also forgot about the shells on the bottom end as theese wear if the car has been thrashed hard on a regular basis

John, what 'shells' are you talking about?

on the crank shaft there is a series of shell bearings normally made of phosfer bronze if theese wear you will hear a knocking sound when under load as the crank will vibrate and start causing damage to the crank itself to get access to theese you will need to drop the sump/oil pan and then undo the crankshaft this will give you access to the shells. if you can read a micromiter then mic up the shells to work out how much wear there is and replace them accordingly with new shells to correct tolerances and dont forget to replace the oil seals at the same time you will have a rebuilt bottom end which will last quite sometime you can also drop out the pistons from the bottom end if you wish and replace the rings with a set of cords piston rings which hone out the bore and clean it up but you would have to run in the engine for at least a thousand miles drop the engine oil and replace with a good quality oil after this and it would be as if it was a new engine

What you are recommending then is a full rebuild. The block must be split to replace those rod bearings.

a rebuild is the best option as a second hand motor would cost more with fitting than you rebuilding the bottom end

Rod knock is a result of the "shells" or rod bearings being worn out. The noise is only heard when the gas pedal is pressed. --- Sometimes engines will make noise from pistons with short skirts. They will rattle a bit until the engine warms up and the piston expands to fill the bore. Pontiac 3100 and 3400 engines do that.

In the Subaru world, this is referred to as piston slap. It's very common on the higher mileage EJ engines.

I just bought a nice used '05 Legacy GT and this happened to me after 2 months. They said it would cost $9.5k to fix which is what I payed for the car. Rod Knock messed up my lower end and turbo with shaving metal. Now im figgidy figgidy f*****

mine does not knock on acceleration,...but, when you let off

always noisy when first start up...then quiet,....when shifting, or downshifting,....it sounds like I have a marble factory under the hood...lol

My Subaru BH5 is making the same noise when i press the gas pedal. kNock sound, and goes on and on..

97 Subaru legacy 2.2 knocks when starts then it stops.