Auto transmission issue

He everyone, 

I have bought a skoda fabia 2001 1.4  petrol with auto transmission recently. But since 3 weeks i feel grinding in my gas pedal when car is shifting in low Rpm to 3rd gear i guess so, but when its in high rpms i don't feel the grinding noise when shifting to 3rd gear. I don't feel it in any other gears. I thought it might be a CV joint but i think its related to transmission as the rpm comes down when gear is changed i feel it then when it increasing the speed and rpm the grinding noise is going away. What you guys suggest?  If anyone else had such problems so let me know

Thanks you