im not impressed with these pk6 gearboxes, they seam very weak and unable to take the torque from the renault powerfull diesel engines, but i would think the very first thing you need to do is check the gearbox oil level is correct, you can either ask your renault stealer to check it for you or i can post up a picture and instruction of how to make your own dip stick and how to check the level, are you able to take the gearbox out and investigate if needed or do you need someone else to do this for you. I took my pk6 gearbox out a few weeks ago due to excessive noise in first and second gear, after stripping the gearbox down i found a very very worn tapper bearing and replaced with a supperior quality bearing and all is well now, there's 3 main shafts in the gearbox and 6 tapper bearing also 4 moving selector rings, if one or more of the shaft bearings are worn this allows the all shaft and selector to move about which can cause the gears not to mesh too well causing excessive noise, one thing i noticed with my faulty box was the gearstick moved backwards when i applied power , does your gearstick move. This gearbox is not too difficult to repair if you can spot the faulty worn item, ie bearing in my gearbox came out in 20 odd pieces lol.