Metal noise in 5th gear

Hi guys, 

About 2000 km ago, i´m starting to ear a metal noise when i put the 5th gear and when i do not accelerate.

When i´m accelerating i can´t ear the noise but when i remove the foot from the pedal i ear a metal noise.

The other gears don´t do this sound.

Skoda Octavia Combi 2.0 TDI (2005).

131000 Km.

Have anyone experience this? Clutch, gearbox problems?

I went to a garage but the guys said that it must do a real diagnosis but this cost $$$. I my problem is i don´t have a trusted mechanic so, dose guys could tell me that the complete gearbox must be replaced, but the problem could a lot simpler.

Cheers,

Bigster