I don't know how true it is, but I was told that there are air filters in the avionics ventilation system, and these are rarely changed ?
Thus, the ventilation fans have to increase their speed to maintain the avionics cooling airflow as the filters become clogged.
It certainly seemed to me that the ventilation in our older A320/321s was much more noisy than in the newer ones, (and that airline did have some aircraft with very low serial numbers).