I've been trying to diagnose this rattle since i bought my 08 civic Si; when the car is ideling and the AC is on there is this rattle in the transmission area; when i turn off the AC the rattle stop, or when i step on the clutch the rattle stop; otherwise when ac is on and clutch engaged there's this rattle, and i can't find the connection between the AC and the transmission ?? i thought about the motor mounts might be bad; however they all in good shape and there's no engine vibration when the car is idling with AC on, any idea what to look for here ?

another problem i'm trying to diagnose is a hissing noise coming from the AC as soon i turn off the car, it is a noise like a tire leaking air or something. it remains for like 5 minutes after turning off the car then it stops. I traced it with a stethoscope to find that it is coming from the AC low pressure line (all over the line); i checked the AC for leak using the pressure gauges and there's no leak and AC works perfectly; any idea what is this noise ?