Intake boot is the big rubber tube running from the airbox on the drivers side to the intake manifold (big black hunk of plastic running the lenght of the engine on the driver's side). As the boots age, they crack and let in unmetered air and cause problems. You need to remove them to fully inspect. Also check the smaller hose that branches off that for your idle control valve.
Your disa plugs into the side of the intake manifold by the intake boots and certainly can make a flapping noise when it breaks. Can be rebuilt or replaced - read about all this in the e46 wiki link near the top of the page.
I ran some seafoam in my SAP passages as a preventive measure after reading that some people had seen their passages clog with carbon/soot and running some seafoam in them cleared them. No idea if it needed it or not, just thought it couldn't hurt. No way to really verify if it did anything other than make a sh!t ton of smoke.
Running seafoam in the engine as advertised is something I'd avoid unless my engine was really high mileage and known to have had a sketchy maintenance history. I hope I'm never in that situation. For really bad cases it may help, but otherwise it's probably a waste of time.
I replaced the hose and its running kind of the same way it use to. Now i heard that if I took out the DISA valve it will actually make your car run better....? IDK but thats what a service adviser from BMW told me. I watched a DIY video on how to replace those valves and it showed how the flap inside the valve could be broken and could actually cause a flapping noise or something. Now since I replaced the sap hose IM pretty sure I could clean it up. Would it be a good idea to run some sea foam in it? I seen a video on it but i also heard that there are some cons from sea foam. The end of the valve where it meets the hose looked a little dirty so I figured I might need to clean it. The guy also told me something about a boot hose or boot intake thingy...? idk but if somebody could direct me the right way I can fix this thing myself since I'm a pretty handy guy. Thanks for all your support