JPA has it spot on, most likely condensation (could be from low grade gas, they have water mixed in from not cleaning the tanks very often)
and it freezes in line, then you drive and it heats up and starts to boil, but its trapped in silencer so the bubbles from the boil make it vibrate around in its little cell.
Try name brand mid grade gas, its not used as often so its cleaner, all tanks in ground get water in them, the more you use that tank,
the more the tanker has to refill which mixes the water with the gas and you get crap gas, so avoid real busy stations or use mid grade, also named gas stations
are required to clean the tanks and change pump filters once a month, they have quarterly inspections, mom and pop or 7-11 type stations dont have that, they only get inspected once a year.
Or it could just be purring cuz its a happy car...
18 Touring CX-3