Yes, probably a rumor. However, this report is sound ;-)
http://www.theaustralian.news.com.au/common/story_page/0,5744,10646764%255E2703,00.html
Sonic boom rattles Pakistanis
From AP
September 02, 2004
ISLAMABAD: A sonic boom that many mistook for an explosion rattled windows and nerves in Pakistan's capital, where tensions were already high in the hours before the government swore in a new cabinet under heavy security.
Pakistanis are on edge after a series of high-profile al-Qaida arrests, assassination attempts and thwarted terror attacks in recent weeks.
Police and emergency officials reported receiving thousands of calls after a loud boom Wednesday that rattled windows across a large swath of Islamabad.
Hundreds of jittery residents left their offices and poured into the streets to see what had happened. The cause of the boom was initially a mystery. The air force at first ruled out a sonic boom. Local media reported rumors it may have been caused by an explosion at a textile factory.
It took nearly six hours for the air force to confirm that the boom and vibrations were caused by one of its jets breaking the sound barrier on a training mission near the capital.
"We did not intend to scare people. The plane was flying over its training area," about 50km south-west of the city, Air Force Cmdr. Sarfaraz Ahmed said.
The boom came about two hours before President Gen. Pervez Musharraf swore in a new 32-member cabinet that will serve under incoming Prime Minister Shaukat Aziz.
Earlier, authorities had announced the arrest of two terror suspects in the southwest of the country in the past few days, including an Egyptian man the government said it considers an important figure in Osama bin Laden's terror group.
The capital has been under a virtual lockdown for months following a series of suicide attacks and thwarted plots against senior officials and the US Embassy. Pakistan has suffered a series of terrorist attacks, most in the restive southern city of Karachi.