(Reuters) - Truck and engine maker Navistar International Corp <NAV.N> reported a fall in revenue for the sixth straight quarter due to lower truck sales and said one of its units had received a subpoena from the United States defense department.

Shares of the company were down 4.6 percent in premarket trading on Thursday.

Volkswagen AG <VOWG_p.DE> earlier this week agreed to an engine technology and purchasing alliance with Navistar and bought a 16.6 percent stake in the U.S. firm.

The net loss attributable to Navistar widened to $34 million, or 42 cents per share, in the third quarter ended July 31, from $28 million, or 34 cents per share, a year earlier.

Revenue fell 17.7 percent to $2.09 billion.

Analysts on average had expected the company to earn 14 cents per share on revenue of $2.18 billion, according to Thomson Reuters I/B/E/S.

Navistar Defense LLC, a unit of the company, said it received a subpoena from the United States Department of Defense Inspector General asking for documents related to the sale of some independent suspension systems to the government.

The Lisle, Illinois-based company has submitted the documents and intends to comply with the subpoena, Navistar said in a filing on Thursday. (http://bit.ly/2cvAtdv)

The subpoena is related to the independent suspension systems sold for military vehicles between Jan. 1, 2009 and Dec. 31, 2010.

(Reporting by Arunima Banerjee in Bengaluru; Editing by Sriraj Kalluvila and Shounak Dasgupta)