But the noise turned out to be a washing machine, the High Court in Pretoria heard.
Sean Rens, manager of the International Firearm Training Academy in Walkerville, said he had many conversations about firearms with Pistorius, who is accused of murdering his girlfriend Reeva Steenkamp on February 14 last year.
He said the double- amputee runner had "a great love and enthusiasm" for guns.
Mr Rens said he met Pistorius in 2012 and trained at a gun range with him. In one conversation, Pistorius described how he was startled by a noise at home and decided to clear the house by drawing his gun and checking rooms, according to Mr Rens.
"He went into what we call 'code red' or combat mode," Mr Rens said. "When he came to the source of the noise, it was the laundry or something."
Pistorius had tweeted about the incident in November 2012: "Nothing like getting home to hear the washing machine on and thinking it's an intruder to go into full combat recon mode into the pantry!"
The athlete claims he killed Ms Steenkamp, 29, by accident before dawn on Valentine's Day last year, shooting her through a closed toilet door after mistaking her for an intruder. Prosecutors say he killed her after an argument.
Mr Rens said Pistorius was aware of South African firearms and self-defence laws that say you cannot shoot at an intruder unless your life is in danger.
In a gun licence test passed by the athlete, one of the questions asked whether a homeowner was allowed to shoot burglars on the other side of a security gate. Pistorius answered "No", Mr Rens told the court.
Ms Steenkamp's mother June was in court for a second time yesterday after attending the first day on March 3. The trial continues.