This is an old question, but here's another way to do it.
You can modify the R code itself instead of the chunk options, by wrapping the source call in suppressPackageStartupMessages(), suppressMessages(), and/or suppressWarnings(). E.g:
```{r echo=FALSE}
suppressWarnings(suppressMessages(suppressPackageStartupMessages({
source("C:/Rscripts/source.R")
})
```
You can also put those functions around your library() calls inside the "source.R" script.