Hi. I have 2008 Mazda 6 (1st gen) 2.0L 105kw diesel. My problems that I noticed:

Weird tractor-like noise when accelerating from 1k to 2k rpm, then disappears, doesn't happen on high rpm;

When slowly accelerating in 1k-1.5k rpm range, the whole car kind of shakes, can feel it in pedals, passengers can fell it as well;

Also, I get blue-ish smoke on cold start, most likely burning diesel, might be a problem with diesel injectors (you can slightly feel the misfire in a cilinder or few(hope not) when idling on cold start, when gets worm, it's fine, as well as with smoke from exhaust), maybe glow plugs(hopefully)?

Also noticed my gear box acting differently after I once put fuel system cleaner into a fuel system through filling hole(dunno how it's really called) and went up to 100kmh on 4th gear. Car started having better power, but my gearbox, when I'm not on a low enough rpm, I can't put it in 1st gear. Googled this stuff, it seems some cars have it to prevent money shifting, but in my car I had been able to do that before. Also sometimes it's harder to change gears.

Any ideas on how serious these problems are and any solutions for them? Maybe putting all these problems to one there might be one, I hope. Want to know how much approximately I will have to pay to a car mechanic. Thanks in advance.