We've got a 1980 Ford tri-axle grain truck on the farm with an 855 cid Cummins in it, and this spring it started losing power, so dad started lookin through it and found that the compressor side housing of the turbo had gotten loose and worn all the vanes off of the compressor! So we got a new Holset turbo and let me tell you, this thing whistles like a SOB! I swear the sound alone adds about 100 hp. It'* darn near an aphrodisiac.
Its not that loud but it is very noticable in the truck. When you start off in like 4th-6th gears, you hear more of the enginge until you reach 1700 rpms. Around 7th and 8th as soon as you hit the accelorator after shifting you hear the turbo right away. Sweet whistle noise. Then you can hear it really well with the engine brake on.