I have an mp3 128 Kbps music file with particular parts where a series of single note only on a cello is played initiated with a bell click. Less than a second after a bell click and cello note playing starts a very faint Morse code like series of bursts in 2 - 5 kHz range for a couple of seconds then fades away.
I was able to localize their frequency range and duration with iZotope RX on a time-frequency spectrum.
They happen only in the parts of a cello note playing after a bell click in some parts even fainter than in the others.
Could they be attributed to errors in mp3 compression or to other factors?