I just got my car, but with the 17" wheels I notice a lot of noise over bumps. I'm going to do a full audio build, and I want to know if there are any key areas to Dynamat? I hear a lot of noise around the hatch area over bumps, but I'm wondering if I dynamat the entire hatch door if the lift supports will still work to hold the door up.

 

"Key areas" should already have factory sound damper. It's not as thick as dynamat and typically does not cover large areas (however does indicate where Scion thought damping was necessary.)

Dynamat shouldn't overload your hatch struts unless they're close to or already shot. Aftermarket struts are much cheaper than OEM. Ebay has several vendors offering lifetime warranty.

 

Generally the doors and trunk floor area are good places to start (especially if you have exhaust drone). If the hatch lifts up you want to use the product sparingly, as even if the struts are in good shape they'll struggle holding the extra weight. It shouldn't blow them out, but speaking from experience on 2 vehicles, the stock struts aren't designed to support much more weight than stock. If you're planning on complete coverage (probably completely unnecessary on that vehicle) then you'll likely want to grab some beefier struts (I'd test with the stock pieces first though before you spend extra $).

On my tC2 I found the wheel wells a fantastic place to deaden, but I wasn't happy until I dampened everything (mostly for exterior noise reduction). My tC2 is nearly 100% covered (lightly on the hatch) in Noico 80mil (much cheaper than Dynamat and just as good) and then has a layer of SecondSkin Heatwave pro over top of it. I added ~38lbs to the car with all the Noico and it was 100% worth it for the tomb that it is now.

 

Doors, roof, trunk floor, inside hatch.

 

Cheaper cars generally still don't have this, and 1st Gen Scion xBs in particular, while supplied with electric locks and windows and such, were devoid of much of the other doodads (like sound-deadening) because they were intended to be blank slates.
They underestimated the demand by other than young customizers. That said, I'm so happy to have found all the comments and information here about sound deadening. My hatch will be the first victim, followed shortly by those nearby access holes with snap covers.

The exposed metal between the flimsy panel and rear window on mine has a lot of bare metal now, as the bike I keep in there most of the time rubbed against it a lot more than I was aware of (even with a towel over it the paint rubbed off).
So Rustoleum or something, yeah, but I want to put something cool over that area. Hmmmm.....

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I built a 4 cubic ft enclosure tuned to 34hz I believe for a single 15L7 kicker and fit it perfectly with no spare room in the back. I can tell you from experience that the trunk lid rattled excessively more than the rest of the car so focus most of your dynamat on the rear liftgate.