I have driven 90/100/110/115/130/150 TDI engines and I haven't ever heard it over the diesel clatter except on my 150. We have 2 passat 130's and neither of their turbo's whistle, neither did the 110 I owned for 3 years. There is a slight whistle on start up, but that's it.
my tdi 90 turbo seems to whistle much more with the tuning box fitted I could hear it before if I pulled away with the window down but it does seem more noticeable now.If it`s not a VNT type turbo (90 TDi engine shouldn`t be) try lightly spraying the actuator arm on the turbo with some light oil and work it back and forth with a vac pump.Mine was stood for a few months before I brought it and this had partly seized causing intermittant power loss and this did cure it.If you know what you are doing you can do some data logging with the vag-com diagnostic tool whilst driving (have someone drive for you while you do the tests) to confirm correct turbo boost behavier under load.
After reading numerous posts on a US website about removing the only box in the exhaust system on my PD115 I performed a mufferlectomy on my car last weekend! Pipe wasn't quite long enough because I didn't think the rear box was so large but it makes quite a nice note when driven and I made a tip so that the gases point down etc and especially when starting the car you can hear the turbo whistle ever so clearly!! It's possible to hear the wastegate operating also when chaging gear, it is only really loud in 1st gear though.
Since dump valves are pointless on TDI's it would be quality if the wastegate operation could be heard a load clearer, any pointers?!?
so i think the answer to your question is yes you should be able to hear the turbo. but as to why its down on power and using more fuel, well if its more of a hiss they are hearing then it could be becuase of a split intercooler or vacuum hose pipe leading to loss of boost.
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