What is this on top of my engine

I rotated the picture and enlarged it so you can see it better. I went out and poked around and found some bees in there. The thing is it looks like it is wrapped in plastic. As long as there is no reason for it being there I will get it out some how.

Yeah the plastic "wrapper" does make it strange but we saw something similar when a rat made a nest using one of the plastic wrapped sound deadener packages that ford often places at strategic areas in the body of the vehicle to absorb road or tire noise. They are a small plastic bag filled with a fiber material, I'm not sure if it's polyester or Fiberglas.

It's amazing to me how a small sound deadener like that can make such a difference but that's why after several years in production vehicles get noticeably quieter than when introduced. All the manufactuers have "NVH" teams that work to reduce "noise, vibration and harshness" in the current models and often the results are substantial.

We removed a nest very similar to your photo from the valley of a pickup's Diesel engine. The truck came in with a running problem caused by the rodent chewing through the injector harness wires under the hood.

Be glad that your explorer seems to not have any of that type of damage. Fixing wiring harnesses is time consuming and expensive!

Weirdest rodent damage I've ever seen is we had one customer with an XLS explorer that had squirrels that liked to chew on the outer edges of his black plastic fender flares! They would climb on top of his tires and chew all along the edge of the flare and ruin it. He finally had to resort to extermination to stop it.

All rodents incisor teeth constantly grow so they have to chew to keep them worn down. Usually their diet has enough nuts in it they have to chew open but I guess some of these "city squirrels" don't get enough of that and have to resort to chewing other things. We had a house fire in my town caused by squirrels chewing through wires in the attic, they're bad news in a house or car.

I finally had a chance to pull this out and it looks like it was supposed to be in there. I has a part number on the end but I have not been able to make it out.

Not a nest

Wow! You learn something new everyday. That is definitely a plastic covered foam sound deadener, you can see it was molded to fit inside the intake runners. It's the first one I've ever seen on an explorer, I wonder if your explorer engine was a test engine and that sound absorber was added to see if it reduced engine noise by a big enough margin to make it worth installing in all explorers? The damage to the plastic bag and foam was probably caused by a rodent that wanted to take up house keeping but the foam prevented that. Maybe you should put it back;-)
Oh BTW, this spring when I changed my thermostat, I found a mouse nest in the valley of my V6 so I started running a mouse trap on top of the engine every night. So far I've gotten 8 of the buggers over a 5 week period but none in the last 3 weeks. I would recommend that when you check your oil you keep an eye out for traces of mice in the valley of the engine and chew marks on wiring and the battery insulation pad (they chewed a bunch off mine) If they're present they'll often come back to your vehicle each night. I use a good old victor snap trap placed on top of the engine. I drilled a small hole in the bait tray so I can wire a peanut to the tray. If you just use peanut butter I've found the little bastards can lick all the peanut butter off the tray without springing the trap! Once I wired a peanut to the tray, it started catching almost every night until I guess I've gotten all the explorer oriented mice. I actually looked forward to checking the trap every morning to see if I was adding to my total. I even though about painting little mice icons on the fender but I figured that was a little much;-)

Sound absorber

That bagged foam definitely was placed in the engine valley at the factory, the pattern of it corresponds to where the intake runners enter the sides of the heads so it will fit there without having to be mechanically fastened. All Lincoln engines since 2000 have that kind of sound absorber in the engine valley to reduce engine noise and "air flow drone" from the plastic intake manifolds. The foam is in a plastic bag to keep it from absorbing oil or water. Now that I think about it (I'm a recent retiree with CRS-can't remember sh**) I might have seen bagged foam in the valley on an explorer limited, the limiteds had all sorts more sound deadening than xlt explorers, they were much quieter driving down the road. The plastic intakes are much noisier than the old cast aluminum intakes so putting sound absorbing foam in the valley is cheaper and lighter (think gas mileage) than undercoating on the firewall. Heck, the reason for plastic intakes is that they're lighter than the cast aluminum, it's all about weight reduction for gas mileage to meet the govt cafe regs for gas mileage. Hope this clears it up.