What do i need to fix so the paper would pull in?

Peel the beginning edge of the paper roll off of the rest of the roll. It will be lightly glued to the roll to keep it from unwinding.
Cut the end piece of the paper if it tears when you pull it off of the roll. You want the edge to be the same level all the way across so that it will feed into the machine easily.
Flip the paper holder bar away from the Unisonic adding machine and slide the paper roll onto the bar. You want the paper on the roll in a certain position. Make sure that when the loose edge of the roll is on the bottom, that it is facing in the direction of the machine so that you will be able to pull it toward the machine to feed it in.
Pull the lever up that is located near the paper slot in the back of the adding machine.
Take hold of the edge of the paper and feed it into the paper slot.
Put your finger on the paper roll-up knob and spin the knob toward the back of the machine. This will turn the rollers inside the machine and push the paper up. Stop turning when you see the paper come through and above the cutter.

Paper release lever is activated

need a little more info. If it doesn't feed when you manually try to roll in the paper, it may be that the rollers have flattened out under the platen due to age. There may be an obstruction or something stuck on the paper pan. If it rolls in ok but the lever doesn't advance the paper it could be that the index pawl mechanism is broken, loose, or a spring is off.

open door, place paper in cradle and pull a tail of paper from the roll with this portion facing down, then close the door, no need to feed it though any rollers etc.
pulls the paper from the bottom so the side that is loose goes towards bottom.
if you hold it in hand, the paper that comes from bottom of roll goes in cradle pulling 3 inch paper out of roll and closing door on the paper. tear off

I highly doubt the main board is going bad. There is a huge difference between an electronic high pitched noise vs a scanner noise. Does the noise only occur while scanning or when parts are moving? If yes, then a scanner noise. If the noise is on all the time even when idle, then you have an electronic noise like an open or leaking capacitor.
On this particular model the seperation idle roller builds up with paper dust and will cause a high pitched noise. Like nails on a chalk board. Very simple fix. Open cover. Grab seperation assembly and pull toward the tray at the front of the scanner, it will pop loose then lift up. Make sure to pop it back into place good. Be forcefull. With the assembly in hand, clean, and lube idle roll. It is the first metal/plastic roll the paper touches at the tray entrance. Also clean the shafts and bushings for the rubber rolls. Reassemble and you are all set. Happens on EVERY I420 scanner I have seen.

Pull about a foot of paper loose from the roll so it's easier to handle. Put the paper from the new roll in the back slot, and hit "FEED" this will feed it through; after you have it through, roll up any lose paper, mount roll in holder, done, Should take about 3 minutes

PAPER ROLL REPLACEMENT
Never insert paper roll if torn. Doing so will cause paper to
jam.
Always cut leading edge with scissors first.
1) Insert the leading edge of the paper roll into the opening.
(Fig. 1)
2) Turn the power on and feed the paper by pressing . (Fig.
2)
3) Lift the attached metal paper holder up and insert the paper
roll to the paper holder. (Fig. 3)
DO NOT PULL PAPER BACKWARDS AS THIS MAY CAUSE
DAMAGE TO PRINTING MECHANISM.

Remove roll pin holding trigger. Examine trigger for wear. A loose/ worn trigger can cause problem that you describe. Also examine the roll pin that holds trigger, if it is worn where trigger rides replace pin. If trigger and roll pin look ok then problem most likely tva6 valve. This valve has a pin that wears out. Order from ebay for under $20.00. If the gun leaks air, remove head and clean orings in the headvalve. Pull cylinder and clean orings to cylinder seal. Remove fine scratches on body of gun where orings contact metal. Use 400 grit paper. Lube all orings with silicone paste (sold at lowes in the plumbing department for faucet orings). To remove tva6 valve remove two pins in body of gun and apply air pressure to pop out valve. Use glove when you catch tva6. Use caution because the valve can come out with force (lower pressure ). Good luck

you may need to 'select' the printer in your control panel. Or the printer driver may be blown.

first the paper is rolled over the top and then roll it slowly under untill you can pull it out by hand. At this point you can close it and it will automatically start to feed.