Hi. I'm a water treatment operator. I would guess it's the chloramines. They are not good in the way of taste and odors. Depending on how your city is ATTEMPTING to treat your water...adding ammonia or ammonium sulfate to intentionally create chloramines (longer disinfection for lower usage pipelines), this can result in ammonia and hypochlorite (bleach) in your water which may be misconstrued in the homebrewing world as a sanitation problem, i.e. lack of rinsing.
The other option here is that your utlity is attempting to breakpoint chlorinate but failing at times. Your water would then be measure in free chlorine (typically mg/L or possibly ppm, same thing by the way) which IMO is by FAR the best option for a homebrewer. The major problem in this scenario is that when/if ammonia is present in particular, and improper dosing of chlorine is a factor, the water can fluctuate back and forth between free residual and the moment before free residual which is comprised of constituents we call dichloramines and trichloramines. If your water has ever smelled like algae or had an "earthy" smell, this may well be the case, even if you are not actively noticing it at the time of the brew.
BTW, for anyone who did not know, if your water utility chlorinates using free residual, you can pour the brew water the day before and leave in a bucket/container with no lid overnight or even for a few days in a fridge. The chlorine will dissipate leaving you with low or no chlorine.