Hissing/scraping sound when engine under load

[Afrikaaner]

So i've had this sound since i replaced my flywheel, clutch, transmission, headers, and cats (all at once). I have no idea if this is normal, but it always worried me slightly, and now that my passenger side cat gutted itself, i'm more concerned.

first a little backstory: when i got the car, there was a burnt coilpack on the passenger-side cylinder closest to the firewall (cylinder 5 i believe). Took me a while to get it fixed, so i put some (not too many) miles on my car while running on 5 cylinders. i was concerned i may have damaged the cylinder/piston or something by doing that, but once i got it working, it seemed to run fine except for a very occasional bout of misses (never more than 1 or 2 seconds with misses) on (im assuming) the same cylinder which hasn't happened in many months now.

fast forward to this winter, i put on all the goodies i mentioned above, and it started making this hissing/grinding noise whenever the engine is under load (almost like a turbo spooling), and now the passenger side cat broke, and i'm wondering if it was due to something related to all this.

and now that the cat is a straightpipe, it seems that the noise isn't a constant hiss/grind/scrape like i thought before, it sounds like it might be coming from one cylinder as in its not constant, it seems to be the same noise repeated very quickly so it sounded like one. today i was driving it trying figure it out, and there seems to be a sudden change in the sound a little above 2000rpm, but that quickly returns to the same sound by 2500rpm, if not sooner. this may be my paranoia, but im starting to get concerned. any ideas about this? i'd really like to at least isolate the noise before i ruin another cat for the same reason. thanks!

[evildky]

is the noise present when at idle in neutral? id the noise present or does it change when you depress the clutch? did you replace the TO bearing and pilot bearing when you did the clutch?

[Afrikaaner]

yea, everything was replaced, and it has nothing to do with whether the clutch is depressed or not, so it cant be the transmission (brand new anyway) and its probably not the clutch, though i guess it could be the throwout bearing, but i used the nissan one that came with the transmission. i did notice today while trying to figure it out that it makes more/a slightly different noise at about 2250rpm and again at like 3250 rpm (i think it was 3250 or so), so do you think it could have something to do with the variable timing or a lifter or valve or something <==me grasping at straws

i dont know if it has any other points where it makes the extra noise, mainly because im not in higher rpm levels frequently enough, but if i rev the engine all the way up (in neutral or with clutch depressed) i think that it makes the same noise as the as the engine is winding down.