Freaking out

[batshapedheart]

Wednesday afternoon I went to my break and when I put the car on reverse a "tak-tak-tak-tak" sound came from what I think it could be the engine. I turned off the car and a minute later I turned it back on, no sound.
I called my Indi the next day and I told him how the sound hadn't happened again neither on the way back to work nor the way back home that day. He asked me to monitor it because I might bring it and we wouldn't be able to replicate it.
It hasn't been until this afternoon that it happened again.

I'm freaking out.

My tranny was just rebuilt back in May and I took care of the IMS then which was not necessarily cheap (7k) so a failing engine would def not be the best of news.
I don't know what it could be and its Saturday so there's nowhere to take it until Monday morning.

Help!