they com in both forms, but this one definitely seems to be made of steel - the experts of this industry site believed it, too:

Tenneco created a thin-wall, lightweight steel muffler that can hush the harshest engines, including diesels. Tenneco used computer-aided engineering to determine stamped irregular patterns of circles, rectangles and beads in the single-wall shell that reduce noise by up to 5 decibels and a welding assembly process that uses fewer production steps. The design has reduced muffler weight by 20 percent and saved 15 percent in steel costs compared with conventional mufflers. The muffler's first use was in 2004 in Europe on the Peugeot-Citroen D2X6 platform. It will be used by GM on a new global vehicle platform.

Englisch term (edited): steel

Wobei du in DE vielleicht besser von Stahlblech sprichst, damit man nicht an Doppel-T-Träger denkt. Es geht ja nicht um die Rohre, daher kannst du ohne Risiko von Stahlblech schreiben.

if metal was used it would work both ways.
Both aluminium and steel are used, titanium is used as well

--------------------------------------------------
Note added at 20 mins (2006-04-28 09:27:26 GMT)
--------------------------------------------------

but in your specific case it must be steel. not aluminium.
Look at their american website. : http://www.xyz.com/news/press/archive/press_2006/04052006-1....