Civic EG

'06 civic dx-g, 1.8ltr, 5spd manual trans, 2door, 171 268km. When I start off in first gear, I hear a chuckling/rattling sound from what appears to be the tranny. Second gear; the sound isn't quite as loud, third gear; the sound is hardly noticeable, fourth and fifth sound fine. Sometimes this happens immediately upon start up, other times I must drive a few minutes until the noise becomes more prominent.
Has anyone out there encountered this problem or have any suggestions as to what the cause is? Thanx