Transposons, Viruses and RNA interference
Transposons and viruses don't "make siRNA". However, an endonuclease called Dicer can process dsRNA from these sources to produce siRNA which is then loaded to the RNA-induced silencing complex (RISC) to suppress propagation of viruses and transposons. The following papers provide a good overview:

Buchon N, Vaury C. 2006. RNAi: a defensive RNA-silencing against viruses and transposable elements. Heredity 96:195-202.

van Rij RP, Andino R. 2006. The silent treatment: RNAi as a defense against virus infection in mammals. Trends Biotechnol 24:186-193.