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I recently bought a non running 95 Z28 A4 Convertible, 108,000 miles. I replaced the OPTI and got it running. The car seems to idle fine and ran ok (but not great) at higher RPMs when I initially took it for a test drive. After it fully warmed up it started "surging" so I took it back to my shop. I hooked up a fuel pressure guage and restarted it. Fuel pressure was good at idle. I tried revving it up to see how that would affect the fuel pressure. When I opened up the throttle it made a LOUD squealing noise that sounded like it was coming from the top/rear of the engine. It sounds a lot like a squealing belt but I am certain it is not the belt making the noise.

What is there at the top/rear of the engine that could be making this noise? I have worked on a lot of older engines but the LT1 and Fuel Injection is new to me. Could the injectors make a noise like this? Based upon the way the engine was running I thought it could be a vacuum leak but I have never heard a vacuum leak that was anywhere near this loud before. Could it be a vacuum leak?

Thanks, Joel

PS. I don't (yet) have a scan tool.
 

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Once my thermostat housing was leaking a very tiny bit, and the small hole in the gasket was shaped to the point where it would make a loud wine. Maybe take a look at your hoses and gaskets - look for small leaks.
 

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My recently aquired 66 GTO made a similar sound, only I likened it to more of a whistle sound. And it was in fact a vacuum leak. I don't know exactly which line was leaking because I replaced and tightened them all at the same time. No more whistle.
 
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