Does anyone know what the little box attached to the hose does? It's missing from mine, and is an obvious air Intake leak. I didn't notice it before I changed my thermostat, and the car was idling fine, but now it runs rough at idle until I cover the hole with my hand. Do I need to buy a new hose with the part on it or is there another possible solution? Here's a picture of the part...
https://www.partsgeek.com/gbproducts/WC/17919-01168222.html

 

Hi, if it's missing the part on the side of the air flow pipe then I believe it is missing the resonator.

 

The resonator does not appear to serve any useful measurable purpose. You can order a suitable air pipe, without the resonator, for the 2006 and up XJ or S-type.

 

Mine is an 04, so I wonder if it will be ok if the hole is securely covered. It seems to run fine then. Something must have been covering it before but I didn't notice it.

 

Yes . "Securely" and "covered" are the key words.

 

XR849560 is the part on both of mine.
An S Type 4.2 part, that fits for about half the money.
No resonator stuck onto it, makes a cleaner looking install to me.

 

There's a hose that goes from the air cleaner to the left valve cover. The place where it connects to the air cleaner hose broke on mine, is that connection still on the one for the S Type?

 

Yes, costs a lot less, & it looks better, without that resonator box.
Bring the number up on the computer, to see the part.