Acoustic resonances in internal flows can cause noise, vibration, and even equipment damage. One geometry prone to acoustic resonance is a deadleg at the point where the main flow turns into a side branch. Here, we report experimental and computational fluid dynamics (CFD) studies of flow-generated pulsation in such a geometry over a wide range of mean flow velocities. Experimental results of the normalized pulsation pressure amplitudes (P*) versus Strouhal number characterized the flow–acoustic field for acoustically tuned and detuned systems and for branch to main pipe diameter ratio of 1.0. The unsteady CFD simulations revealed the characteristic volume integral of the cross-product of the vorticity field and the velocity field in the vicinity of the T-junction which helped in quantifying the acoustic source power for different conditions. In addition, the cross-product in the direction along the main line is almost exactly out of phase with the acoustic velocity at the T-junction. The pressure amplitude at the closed end of the deadleg reaches maximum when two conditions are met: (i) the system overall acoustic resonance frequency matches closely the frequency of the oscillating component of the cross-product characterized by a Strouhal number and (ii) the deadleg length is tuned to an odd number of ¼ wavelength of this frequency such that maximum acoustic velocity is reached at the T-junction. Synchronized images generated from the unsteady CFD simulations revealed valuable insight into the velocity and vorticity field in the region of the T-junction in support of Howe's acoustic source power equation.