I always suggest the BBC's "In Our Time" with Melvin Bragg.

Some of the guests are really top notch historians and all of them are very knowledgable. Check out their website for an archive that you can listen to online or subscribe on iTunes/Google Play. They have episodes not just on history, but also culture, religion (both of which have history elements) and science. Definitely worth checking out. Most episodes are ~45min, though older ones are more often ~30min.

I haven't checked it out yet, but there is an AskHistorians Podcast that you might want to try. It's linked in the sidebar if you haven't noticed it before.

Although the audio quality is variable, New Books in History has a number of podcasts from current historians about recent publications.

If you haven't heard it yet, Dan Carlin's Hardcore history! It's incredible.