Does your 07 use engine oil ? Are you the original owner ? If not , maybe the PO ran it low on oil and damaged the main and rod bearings....
I hope not, but it could be a possibility. Lots of threads on the JK forum about loss of oil and later loss of engine.
At 1500RPM there may be enough oil pressure to quiet things down.....
By the way, I "cured" my 07 from using oil..... I changed to Castrol Extended Milage Vehicle oil 10W-30 ( green Jug ) It's a syn. blend....No more oil usage....
07-JKU-X /Poison spider Evap.Skid/Master craft grab handles/Or-Fab spare tire-can carrier/ Gibson SS Duals/ Q-tec Floor Liners/Hidden Hitch Receiver/ AntennaX 7inch/ 48" Hi-Lift./ 8"X16"X4"bs Steel smoothie wheels/ 265X75R-16,Kelly Safari TSR tarz.
Viet Nam Veteran 69-70, Troop "D" (AIR) 3/4 Cav. ,25th Inf. Div. CUCHI RVN. UH-1H Crew Chief of the "Pinball Wizard" 68-16224. Call Sign "CENTAUR".