Car started started loosing power and making funny noises 1 month ago, had someone check it out and it was three quarts low on oil! Had oil change done at time, now a month later same thing happened again,, only this time it was 4 quarts low and I have no visible oil leaks! WTF is happening!

knocking noise! ( a cry for help, is ping/detonation boom bad)
never ignore that.!
get the car serviced, is the answer,
yes low fuel pressure on any boosted engines, can crack pistons or far far worse.
the check engine lamp is the warning, ! pre, life or post.
that is its job, called stores DTC< diagnostic trouble codes
and we scan it.

So being it doesn't clatter in electric mode, a safe assumption is the noise is with the gas motor.

Usually a clattering noise is valvetrain noise, like a lifter not pumping up.

Low oil will cause this.

3 quarts low would probably definitely cause this!

You need to find out why you lost 3 quarts in only 80 miles!

Blown head gasket.

most vehicles might be on side of drivers door or has around the front of the radiator or under the hood a sticker that well display oil filter type how much oil what type etc.now look in your owners manual or go on line look up make model of your vehicle oil specifications it should give all the info you need very important the wright oil filter pcv valve all v-8 are 5 quarts.6cyl can be 4 to 5 quarts.if engine is not using oil running normal than do this drain old oil replace pcv valve replace oil filter pour in 4 quarts turn engine on wait about three min turn engine off wait let oil drain back down to oil pan check oil if dip stick reads normal or maybe one third low add one third and save your fifth can.incase you need to add later.every three months oil change is recommended in any vehicle every six months check transmision fluid,every six months check differential fluid especially if you own a 4 wheel drive. that is your front and rear axles.all ways remember oil filter itself well hold about one third to a quart of oil depends on the size of the oil filter.thats why your dip stick my read a little low after checking when your oil change is done.

Sounds funny, I would take it to an indepened repair shop and have them check it out, we have many customer's with Yukon and have not heard of that, I have a GMC with over 315,000 miles and between oil changes, maybe a qts low, have it check else where.

something is strange here thats for sure ,firstly take thermostat out and put it in a saucepan of water and boil it to see if opens .,check the oil light switch and pull the rubber boot cover back to see if any oil is behind it ??.-You are buying trouble with a car with such a high mileage on it ,never buy anything with more than 50,000 miles or you are just buying someones rubbish

I show the capacity as 6.5 quarts, but owners report that 7.0 quarts with a filter change leave the level right at the top of the dipstick. Hope this helps.

I would guess that a lifter is kind of sticky after sitting so long.The cars engine may not of had enough oil and filter changes before you got it.
I was working at a auto shop one time and this guy brought in this car thats engine was makeing noise and even smoking some. The boss told me to make sure the engine was a quart low on oil. So I drained about 1.5 qts out. The boss gave me a quart can of Marvel Mystery Oil and told me to put all of it in the guys car.He told me to start the car and let it idle for a couple of hours, checking once in a while to make sure the car was ok. The car sat idleing and slowly stopped makeing any noise and the smoke almost completely disapeared.
Marvel is a heavy detergent that kind of flushed the engines dirt into the old filter. I then did an oil and filter change.
If I didnt see it I would not beleave it.I hope this helps you.

THE OIL CHANGE INDICATOR WILL NOT CAUSE THE SES LIGHT TO COME ON.THEY MAY HAVE LEFT SOMETHING DISCONNETED NEAR THE AIR CLEANER HOUSING WHEN CHECKING THE FILTER.IF NOT HAVE CODE READ(SHOP/PARTS STORE)