We show that excess noise in micron-sized samples fabricated from Si-d
oped GaAs/GaAlAs quantum wells and heterostructures is a strong non-mo
notonic function of temperature with several sharp peaks below room te
mperature. The noise power at the peaks exceeds the background noise b
y several orders of magnitude. The observed behaviour represents a cla
ssical noise source where only a single type of switching defect with
a well-defined activation energy is present. We attribute the resonanc
es to deep metastable donors and provisionally identify one of the don
ors as the known DX-centre in GaAs and GaAlAs.