To echo Mikko, the mic will always pick up *something* on Auto. If there's very little sound, it'll ramp up the Auto Gain level in an attempt to pick up something, and it'll undoubtedly start picking up some noise from the tape-motor for one thing.
So, Floris, listen to the noise you're getting and if it's tape-motor it'll probably be about the same frequency either on internal mic or external mic. you should be able to get rid of this using your edit package or a sound program in post-production.

Thanks Mikko and Stu. I have done a project now with the new microphone and with lots of music and people talking it does a really good job indoors. And there is absolutely no noticeable hiss/noise or something.

I have just ordered the Rode NT-3 hyper cardioid with On-Stage microphone stand as I want to make sure my sound indoors will be superb. While on it, I also ordered the Rode Deadcat and a Sennheiser HD-280 Pro headphone.

I think I am more then settled with my audio equipment. Looks like I am ready to start shooting this documentary in a few weeks from now.

Furthermore, once I am done with my school, I will start doing some tests with differenct microphones and with the Sony Y lens and other gear, compelte with pictures, to give back to these boards what they have given to me!

Thanks everyone for sharing your feedback with me.