[mstoykov]

Hello everyone,
I recently bought a 2011 Hyundai I30 1.6 crdi 85kw.
Twice until now, when braking hard at about 50km/h, I heard a loud noise, as if something hit the car somewhere between the gearbox and the engine. The first time was 2-3 weeks ago and I thought it was from the pothole I hit with my front wheels, the second time was today on a perfectly good paved road.

I stopped and tried to see if anything was broken (looked for some debris, under the car, under the hood) but everything seemed fine. Then I continued driving and had no problems with the car but now I can't get these incidents out of my head , I keep thinking what it could be.

[TerryT]

I'd be jacking up the car (and supporting it on car stands!), getting underneath and pushing-pulling components north-south, east-west and/or putting a spanner on components to test for tightness to find anything loose...especially exhaust system (e.g. off its rubber donut/hanger,) suspension, sway bars, shock absorbers/coilovers etc.   

[The Gonz]

I sometimes get a horrible grinding noise when ABS kicks in under heavy braking. That may give you somewhere to look.

[mstoykov]

Yesterday I jacked up the car, tried to move everything that seemed to be movable, everything was fine.
I tried to repeat the incident but nothing happened.
Мaybe there was something on the road that bounced or something in the trunk.
 I really have no idea what else to check, everything seems ok. I hate when things like this happen - is it the car or just me panicking

[TerryT]

Thanks for the feedback.  Sounds like you've done what you can, so imo it's wait and see.