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Problem found in versions r29044-110113 to r29143-110126. I could not test 3.7.1, because it did not recognize my 16 GB Transcend flash card (fixed in above versions).

At the beginning of tracks and during rebuffering of longer tracks there is a short humming sound (like a bumblebee) mixed with a fast sequence of random high frequency tones. The sound differs as a function of data format and content. It is very homogeneous with WAV tracks (only humming) and different with lossy and lossless codecs. The same file encoded with FLAC and AAC leads to different short distortions. It is only audible for tracks with initial silence or very dynamic content, for example classical music with quiet passages or audio books. With sensitive headphones, this can be quite annoying.

The original firmware does not show this behavior!

It only happens while reading from the SD card, not from the internal storage.

Initially I thought this might be a CPU throttling issue. But now I think it might also be related to the controller driver.

There is a thread on Hydrogenaudio with independent confirmations of the issue:

http://www.hydrogenaudio.org/forums/index.php?showtopic=86306

If I can provide any additional information to help, please let me know.