Several problems with kit; Underextrusion; Sound level  

Several problems with kit; Underextrusion; Sound level

Hey all,
I've recently aquired a Prusa Printer. I'd like to ask for a help with a few issues.

* Firmware is from around 5th of may or so.
* Belt tension is y260 x270
* Self test okay
* all steps described in manual done
* Multiple contacts with live chat, which helped me with previous problems; not with this current one
* Most of the prints are okay on x/y axis, and mostly on Z, see below.

Tl;Dr: I have some other problems that I am aware of; they are rather not related to the current issues
For starters; I'd like to note that it is slightly damaged - top rod of X axis is scratched along the length, and bearings of X might be a bit off as well. This can be a suggestion for the Prusa manufacturers - as it turns out plastic parts that the rods slide into were in fact sealed despite the groove at the bottom (Groove was not cut). When rods were pushed in; An air cushion prevented them to slide till the end; It created this V shape of the X rods; Which translated to the V shape of the Z axis. When extruder was near the print bed; it was okay, but as the Z moved up, the separation caused by the V shape was more pronounced. It was a half a centimetre or less off; But since it was clamped both at the bottom and at the top, it created the tension which cased Z axis to skip, and for the rods of X to go misaligned even more - which cased bearings to scratch the rod.
This problem in particular seems to be solved; And since I've sanded the rods a bit; I see NO problem with X axis at the moment.

Currently I have three problems:
1) Printer is really loud
2) I have severe underextrusion
3) Stringing on the small models is very pronounced
https://imgur.com/a/jL2x5Pu

Ad. 1: I seem to have heavy vibration causing resonance in the whole printer. I cannot pinpoint the origin, but it is somewhere around left z axis motor. Sound itself is produced when Z axis is moving up and down
Sound (and vibration) is more pronounced when extruder is near left side; It dampens to the point of "really quiet" when at the other;
Sound is more pronounced when Y axis is closer to the front, it's a bit quieter when Y axis is further.
Resonance is felt around the both Z motors; And it produces additional sound with the LED box (Resonating plastic parts).

* I've tried to tighten the frame, and motors, and motors covers to no avail.
* I've tried placing a bit of tension on the frame (E.g pushing it a bit) to test if some loose connection is causing it; to no avail.

It's really irritating...

Ad. 2: To print more or less properly; I have to set speed to ~80% and filament flow to 125%. I'm printing with prusa PLA, default settings. Problem is especially noticeable when doing infill (Fast, broad strokes) but it can be even seen when doing z-adjust (Not every little bit is placed). When testing for the problem, all settings were at the default.
It seems like the filament just stops flowing for a second. As for this moment I have
* Nozzle was cleaned; Extruder was taken apart and reassembled; Tension screws were checked.

Ad.3 nuff said; see photos