General Strange power lost and noise on 3rd gear

[unoonu]

I'm not good at mechanic, my Uno 45 ( year 2003) start making something strange : when on 3rd gear it starts loosing power and make sudden shocks with bangs in the exhaust.....this is very:bang: . all disapear when on 4th gear.
Can someone please help on this, i brought it to mechanic...no solution.

Thanks in advance

 

[unoonu]

Any Help ! ! !