Do I have an exhaust leak?

[general bacardi]

On heavy acceleration I hear a fast "fft-fft-fft-fft" sound. Doesn't sound like mechanical tap (like a sticky lifter) or knocking/pinging sound. Can't really hear it at idle, cruising, or moderate acceleration, but I have a lot of wind noise from my passenger window/mirror, so I might just not be hearing it above 40mph.

I inspected the engine and exhaust and saw no telltale signs of an exhaust leak like rust holes or soot around a pipe junction. I slowly poured 4 oz. of SeaFoam into the PCV valve, waited 5 mins, and started it back up. Plenty of smoke poured out of the tailpipe, but I could not see any coming out of the exhaust manifolds or other places in the exhaust system.

The engine looks pretty clean, but there is some black sooty/sludgy stuff on the intake manifolds where the fuel injectors are located. It's not acting like it has a vacuum leak, it idles very smoothly.

This is my first Subaru, so I don't know if this is just how an EJ22 sounds normally, but it doesn't seem right. Are there common sources of exhaust leaks? I'm wondering if there's some gaskets that commonly wear out and need to be replaced.

[NuwanD]

1st place i'd check is the donut between the headers and intermediate pipe (non-turbo) or the downpipe to intermediate pipe donut (turbo)