wrxflame, I'm sorry, but there just isn't time to explain the various noise and attitude changes that will occur in the minutes after take off - the guys up front are busy!
As you may have gathered in this thread, what happens is that the aircraft uses a high (not necessarily maximum) power setting to achieve the take off and a relatively steep but slow early climb to a procedurally-determined gate height (1500 feet at Heathrow for most aircraft). At that point, the power is reduced to a more neighbour-and-engine-friendly level, and the aircraft is accelerated to the ATC speed limit (250kts or minimum safe speed, whichever is greater) while the high-lift devices are retracted into the wing. As you can imagine, achieving that acceleration on reduced power means a significant attitude and noise change, which can feel like the aircraft is actually descending!
ATC may then require us to level while still maintaining that relatively-slow 250kts; another power reduction and a further lowering of the nose is then required. The next clearance will be to a higher altitude, so the power will be returned to its climb setting (which is quite high), and the aircraft will be rotated to a climb attitude. This transition from level flight to a climb and back to level flight may happen several times at busy periods, and explains the strange noises and attitude changes you experience.
The human body was not designed to operate in three dimensions, and the inner ear (your main motion sensor when sight is not available) is easily fooled into convincing your brain that certain things are happening to your body that actually are not. Being in the passenger compartment of an airliner is an ideal environment for these false sensations to be experienced! Please understand that we don't make any of these manouevres for our amusement, or to discomfit passengers.