Kind of a goan or low moan

Searched TW and tried different suggestions from other posts to narrow it down but can't find it.
I have a low groan or deep moan sound maybe a deep vibrating plastic buzz.
It is slightly there under load, if I put the truck in D start to move the sound is there both cold morning or after warmed up. It is also there when I'm on a slight incline from a stop, not moving forward but under load to keep it on the hill. Again not moving.
I have jacked up the front end and did the wheel spin for sound no clicking or groan sound. No movement when grabbing the wheel at 12 & 6 and pulling in and out. Sound is there even when switching to 4wd when I do hear it.
It's not there under firm push on acceleration. I also mostly notice the sound when in D and the transmission seems to be in high gear, this is at 45mph ish. If I come to a stop and sit there when something kicks in (I assume it's the clutch fan) the sound is there. Checked inner fenders, bumper, licence plate, grill, hood etc. Changed out tires and still there. Truck is 2007 DCLB, has no lift and basically bone stock except for safari snorkel. Had snorkel prior to sound. Could it be clutch fan? Could it be low trans fluid?
Well guru's any ideas? Thanks