The majority of diesels might be too deep a tone for your critter,
but the galloping Goose sound is pretty ubiquitous to small (in railroad terms) IC motors,
and I've used it in both the Soundtraxx and Loksound versions,
for my critters and other IC engined locos.
Also the Sooundtraxx Tsunami 2 Diesel Baldwin,
and other sound file ID 885004 offers a fairly " light" sound,
and I'm about to install one in a Bachmann On30 rail bus
For me, the Loksound Goose version is superior to the Tsunami,
and their decoder ID is ESU 74461 LokSound V4.0
But it's a personal thing and you may think differently.
ESU seem to be reviewing their soundfile,
but I obtained my last decoder from EUROLOKSHOP.com