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Discussion Starter · #1 ·
I had my 6-month-old GLA 250 taken into service because of a sound I hear when letting my foot off the gas. It's hard to describe, but it's sort of like compressing a squeaky toy that has had the whistle removed. Sort of a mild hiss/whistle sound. Faint enough that it can't be heard when the audio is playing.

The dealer said all they heard was normal turbo blow-off. I'm wondering if anyone else has experienced this and if the dealer's explanation makes sense. It's not a big enough problem to be bothered much about so I'm not likely to pursue it further, but am curious if anyone else has experienced this.

As a side note, I had a 2015 C-class as a loaner. It's a very nice car but I'm glad to have my GLA back. :)
 

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Having owned many turbo engine cars, the sound you described is indeed (most of the time) associated with (1) turbo plumbing [some manufacturers actually put in 'mufflers' within the turbo system to reduce the sound] and/or (2) BOV. IF the sound becomes very noticeable with loud hissing and power drop off then there is a leak (clamp hose too loose or a split in the turbo related hoses) and of course that needs to be corrected.

Clarification Edit: BOV (Blow Off Valve) is a generic term for a pressure relief valve BUT there are two very different methods of the pressure relief function:

-Blow off Valve that vents pressurized air from the turbo directly to atmosphere when the throttle is lifted or closed OR if an over pressurization occurs. When the BOV is actuated there is a noticeable hissing sound due to the pressurized air being venting from the turbo to the surrounding air.

-A Compressor By-pass Valve [CBV] (also know as a diverter valve [DV]) function like a BOV, but the big difference is that instead of pressurized air going to the atmosphere it is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor. Some systems (such as Audi) even direct part of the pressurized air back to the turbo to keep it spooled up after throttle lift. The system on the GLA is a CBV set up.
 

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Discussion Starter · #3 ·
Thank you for that SSBGLA45, good to know.
 
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