The fans, by default, are going to be reactive to system temps, which will vary depending on the CPU load usually. Are you running anything that might spike CPU activity sometimes, resulting in the fans kicking up a notch?
You can go into the BIOS settings and change the fan mode to "increased cooling" (or whatever the option is on the G7... on the Gen8/9 boxes there's "increased cooling" and also "max cooling"). It makes the fans run a little faster all the time but at least it won't go up/down all the time based on what's running.
You should also make sure the latest firmware is installed. I don't know if that's an issue, but some firmware release notes for the BIOS, array controller, and ILO have mentioned fixing fan speed problems, and it's a good idea to be on the latest anyway.