Bad Engine Rattle Options

[KillerCelt]

Hello to all the 6gen Celica owners!
Our numbers are dropping and unfortunately, my 99 GT has nearly joined the fallen.

So to describe what's going on:
Took the car off the road after it developed a nasty sounding rattle/knocking sound. Oil indicator was not on but it was low and the oil smelled burnt. I had lent the car to my now ex for a bit and didn't do my due diligence and confirm when the last oil change was done on it and used the sticker from the shop as guidance which was misleading. So it was about 5,000 since its last change and about a quart low. Entirely my fault and I beat myself up over it as I should. There's no excuse.

I had it towed home and now I'm trying to see if this is something that can be fixed or if I need to replace the engine. At idle it does not make the noise. Around 2000-2500 rpm the rattle begins and is present at higher rpm along with a "bucket of bolts" sound. When returned to idle, the noise ceases. After sitting at idle for 5 minutes the noise returns intermittently but is mostly absent. Asking around and searching the forums, I'm trying to figure out what it might be. A mechanic I talked to thinks it could be something with the lifters but doesn't have the time or space right now to look it over. Are there any steps I can take to try to fix this on my own?

Thank you for any help or guidance anyone has on this.

[KillerCelt]

I've heard certain additives like JB Oil Treatment and Liqui-Moly 2009 can help if it is a matter of sludge/lifter issues, but I'm always leery of those "miracle cures." I suppose at this point it's unlikely to make it any worse than it already is...

[Box]

Have you checked oil pressure with an actual pressure gauge yet? Sounds like oil isn't making it to the top of the motor.

[KillerCelt]

No. I'm out of town for work right now but I'll be home this weekend and will be able to work on it then.