We have analyzed a 90 ksec long observation of the bright nucleus of M33 made with Chandra /ACIS .
We detected low-amplitude ( \sim 10 \% ) highly significant variability on timescales of \sim 5000 sec .
We also find associated spectral variability .
The two main spectral components ( a power-law with \Gamma \sim 2 and a multicolor disk -MCD- with kT \sim 0.9 - 1.2 keV ) vary in relative flux .
The MCD temperature also increases with increasing MCD flux .
The pattern of variability is reminiscent of ( but not identical to ) galactic black hole binaries .
A \sim 15 M _ { \odot } accreting black hole may explain this source .