Air carrier flight crew reported unusual noises and vibrations but had no malfunction indications so they returned to their departure airport.

Narrative

While operating flight ABCE IAH to ZZZ1; we encountered metallic grinding sounds and vibrations in the flight controls as well as under the cockpit floor boards. I was pilot flying while the captain was pilot monitoring.We had our first encounter with the suspect noise and vibrations while initially climbing out departing IAH. Shortly after I called for gear up at positive rate and began my turn toward TAPPS; there was a moderate vibration in my rudder pedals and control column. The sound was almost like a bicycle chain slipping while changing gears. I asked the captain if he felt and heard that; and he replied yes. We began to think of possible explanations; doing our best not to make rash decisions as we were in a critical phase of flight. We thought maybe it was the nose wheel snubber or a flap related issue since we had just manipulated those systems. A flight control issue seemed unlikely at the moment since we had performed a successful flight control check while taxiing to 15L prior to departure.I asked the captain if I could climb a steeper rate to increase our altitude in the event we had a major issue. Being higher seemed the better should. He agreed and also asked me to make small turns left and right to see if we could recreate the noise. We did not feel or hear anything. He proceeded to contact the flight attendant to see if she noticed what we had; and to our surprise she did. She could hear the noise and felt it under her feet. While seated in her jump seat.We began to. believe gear and/or flaps had nothing to do with thwart we felt and heard and began to believe it could now be flight control related. As we leveled off at FL270; the plane noise returned and the vibrations occurred. We both agree we needed to head back to Houston. The captain contacted the flight attendant and gave her an special procedures; and she added that while she was toward the back of the aircraft she could hear and feel it back there. We received a climb to FL330 and proceeded. Again while leveling off at FL330 we felt and heard the anomalies again. We proceeded to scour the QRC;QRH for any possible checklist to run. We found nothing related to our condition. I decided to fly close to. max turbulence penetrating speed. In the event we did have a catastrophic control failure so that we would have enough airspeed to React in the event an uncontrollable climb was entered or and uncontrollable descent. My main focus was positive control and structural integrity of the aircraft. We received pilot discretion; descents all the way to the approach gate where we conducted a visual approach backed up by the ILS 26R. The remained of the flight was unremarkable and uneventful.I believe our efforts as a crew; positively impacted the outcome of the flight; and safety was our main concern and can be recognized in our decision making process. The only other landing opportunities we had while inflight would have required us to put unnecessary flight control pressures on the aircraft. We believed that the few minutes saved in arrival time was not worth the excessive strain the aircraft would have been under in an excessive rate of descent to a closer divert airport.