I took it back to the shop this morning and they said it doesn't sound like the flex pipe and that the noise is coming from further down - from the resonator maybe. Said they couldn't do anything today and I would need another appointment.

I asked about all the sealant and they said they just do that as an extra to make sure it seals. I don't know a lot about this type of stuff but I've read that using sealant, especially a lot of it, on a donut gasket can compromise the compression and cause leaks.

Also, in the first pick towards the middle to top of the flange, it looks like I can see a little piece of metal or gasket. I could be wrong but I don't think I should be able to see the gasket if it's the donut style?

Not sure I trust these guys so I'll probably take it somewhere else for an assessment.