Just had new fuel rail and thermostat installed at the dealer. Now I'm hearing chirping sound from engine bay while ideling. Can't hear it from interior only while outside of car. I'm taking it back tomorrow. Any thoughts on what may be causing it?

 

Just a guess. Might have gotten some fluid on the drive belt when changing the thermostat. Try a finger full of silicon paste on the drive belt, not a lot just in one spot. The running engine will disperse it. Don't apply paste with the engine running. Shut the Jag off then apply the paste.

 

Even easier, just squirt some water on the belt. If the noise disappears, you've found the source.

 

Once the belt is contaminated with coolant, often the only permanent solution to the chirp is a new belt.

 

Sorry, don't agree with this. That may make the chirp more profound. Or, make it chirp if that wasn't the noise to begin with. The silicon paste works, if it's a belt noise.

 

Any outcome will indicate whether the belt was/wasn't the cause.

 

There is also a TSB on this for earlier Jags.
They replace a pulley and belt