Aggressive car vibration at 1700-1800 rpm

So my girlfriend bought a used car, a Hyundai i20 2011. 1.4l 55kW diesel motor. She bought it with a guy who is a mechanic, so I was expecting the car to be OK. Once I got to try the car out, I was unpleasantly suprised. So this issue is pretty hard to explain, but I’ll try my best to do that.

So while the car is in neutral, 1st gear or 2nd gear, everything is fine. Also while switching gears at high RPM (above 2000) everything is fine in the 3rd and 4th and 5th gear as well. The strange behavior is hard to explain, but here it goes:

For example, let’s say I’m in the second gear, switch to the 3rd gear, and the RPM is now at 1400-1500. Now when I press the gas and accelerate there is no vibration, but as the RPM goes up, I’ll get a 1-2 second aggressive vibration (can feel even the dashboard vibrating) when I’m at 1700-1800 RPM. As I continue pressing the gas and RPM keeps going above 1800-1900 the vibration is not there anymore. The same thing applies to switching from 3rd to 4th and from 4th to 5th.

Basically it happens every time at 1700-1800 RPM when I try to accelerate somewhat fast.

However this issue is NOT there when I accelerate slowly.

What I think it’s not:
1) I know that cars with manual gear shifting vibrate while accelerating at low RPM (1100-1400). But this is not the issue. This car also vibrates mildly while accelerating at low RPM, but that is not the same kind of vibration that I’m talking about. The vibration that this post is about is more aggressive and shorter.
2) I don’t think it’s unbalanced tires or something similar, because, as I mentioned, the car does not vibrate if I switch gears at high RPM. This leads me to think that this might be something related to the engine or something like that.

One mechanic said it might be the engine air filters, so he changed them, but the issue didn’t go away.

I’m now thinking of taking the car to a more qualified car repair shop, but I want to be able to give them any extra info and possible hints, because I really want this fixed.