Strange sound when i shift to any gear

Hi there ,

when i drive my palio at any gear while the rpm is at 2500 - 3500 there is a strange regular sound which feels like something is rough , something is not running smooth , this sound is present at any gear but it is more obvious at the fifth than the fourth than the third etc , maybe cuz i can keep the engine running at the mentioned rpm , anyway ,
first i thought it might be the engine , BUT , when i shift to neutral and i increase the rpm to 2500 -3500 , there is no sound , the rough sound disappears , the engine runs smoothly .

THE SOUND APPEARS ONLY WHEN I AM ON ANY GEAR , NOT A CERTAIN ONE , AND IS NOT PRESENT WHEN I SHIFT TO NEUTRAL AND RAISE THE RPM AGAIN.

the sound is like : tch...tch...tch.. sometimes its volume is high , other times it is low but it always there .
so, what is this ?? is it the gearbox or the clutch or what ??
This sound is annoying especially on the highway , it keeps going and going and going

thanx 4 ur care

there is something i'm thinking about , the CV joints in the car are making a loud clicking sound while turning , could the damaged CV joints cause the sound i mentioned in the original post ??
N.B. if you concentrate to listen to the sound , u'll notice that the sound appears only when accelerating or driving at a constant speed , while it disappears when the car slows down when i lift my leg from the gas pedal .
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well, this is a unique one. how many kms/mile shave you done already? secondly please check if the clutch is gettign enaged/dis engaged properly.

the car milage is 68000 KM , about the clutch and changing gears , every shift is fine , no problem in changing gears , but once i had a problem in getting the reverse gear , so i went to the service center , the problem was solved by just adjusting the clutch from under the hood in a couple of minutes , the engineer just made the clutch pedal a little bit high , that's it , but then he told me that the clutch is somewhat heavy and also it will need to be changed at about 80000 KM , about the acceleration , it is normal ,
I changed the gearbox oil but no improvement .
there is somrthing else , the spark plugs are now working for about 40000 KM , does this have any relation to the strange sound ?? ( i was once told that old spark plugs can cause high engine noise )