how can i tell.........???

if a coop has been modded???

the yellow beast i am looking at has a K&N and a dump valve.

coupled with serious turbo lag and a slight rattling sound on deceleration. kinda like a light spring on tin rattle sound. does a duff actuator make this kind of sound ?

any ideas ? im kinda

Is there any smoke coming from the engine when warm? If there is what colour is it?

Sounds a bit dodgy. You do get a bit of lag on the 20VT and it is sluggish off boost but the lag shouldn't be that drastic. The best thing to do is test drive a straight one so you can compare.
The rattly sound I'm not sure, could be many things. You'd have to locate the rattle. Does it not make the sound if you rev it when stationary?

I agree with Pinin. The only thing I can think of for that noise is if the dump valve hasn't been fitted properly, or is broken, you get wastegate chatter, which is like a (very fast) chuff chuff chuff sound.
Whereabouts are you? Perhaps you caould get a knowledgable local to have a look at it with you?
H