The ducting likely needs to be replaced and properly sealed around fan and outlet vent. The expanding sealant they used at our house came in two canisters that had to be combined, not just the foam in a can stuff. I'm not sure what it was specifically, looked to be pressured canisters essentially mini-propane tanks.
Edit: Surprisingly easy to find. He wasn't kidding it's expensive!
https://www.canadiantire.ca/en/pdp/f...-0642693p.html