Starts out low then louder
Car makes whistling sound about 5-10 seconds then may not hear again for 10-15 m on 2004 Jaguar X-Type
does it make the noise at idle in park or in gear or when driving down the road?
Had same issue.could be one of 2 things. Check vaccumm line that connects from top of intake manifold to master cylinder. Mine for some reason appeared to corrode. Or check both of the bank o2 sensors located on passenger side of intake manifold (black round disk looking things) orings go bad. Good luck!