You can refer below link for GPU installation guidelines for T620. Fan 5 and Fan 6 are required for configuration with GPU [URL]  It is expected to have higher fan speed when additional cards are installed on the server especially GPU. Did you have latest iDRAC and BIOS FW installed on the server?
Hello, I have installed an Nvidia Quadro K4000 GPU and since then the usual 9-10% fan speed has raised to 35%. I read that having the front fans would be necessary for GPU installation (however this GPU already has active fan cooling on it) Any ideas how the server fan speed can be lowered to the previous 9-10% as before, without me needing to purchase the front fans? Thanks, Istvan
PowerEdge T620 louder when GPU installed