It sounds like it is coming from both sides. When driving it sounded like it was coming from the passengers side and when sitting in that seat it sounded more like if was coming from either the middle or drivers side.
Well I bought the 99' and the noise would come and go. I called the dealer and they mentioned some factory mounts that might be causing it. I pulled the wipers and cowel and there were two orange mounting tabs that the windshield rests on at the factory before final glue in. The dealer mentioned that sometimes they are the cause and that they are usually removed before final glue in. Well I cut them out and the noise is now gone!! It took about 30 min. to do and was fairly easy.
Mine was also dependant on ambient temperature, but it always seemed to squeak, just more or less at times. The problem with using silicone spray is that the squeaking is behind the wiper cowl at the base of the windshield, so you have to pull it off to gain access to where you need to spray. The dealer said that they remove the tabs when a customer has this problem so I just did it myself and saved the labor fees. They mentioned having to remove the hood(bonnet)as well but if you are patient the cowl can be removed without having to remove the hood. The vehicle has been squeak free ever since doing this.
I had a similar problem with my '99 Disco ll and it happened to be that I was running on very low coolant levels.When the heater/air in the auto.temp. control finds low coolant,it tends to sound with a little rattle.Try filling up the reservoir to the top "full" mark and see if it dissapears.