Best DSP algoritms for ultrasonic background noise cancellation
One way to do this is to look at modeling your signal:
$$
x[n] = x_h[n] + x_n[n]
$$
where $x_h$ is the hissing sound and $x_n$ is the noise.

If you can say that $x_n$ is modeled as:
$$
x_n[n] = \sum_{k=K_1}^{K_2} a_k \sin(k\omega_0 n + \phi_k)
$$
where $K_1$ is the lowest harmonic of frequency $\omega_0$ that makes it through your high pass filter, $K_2$ is the highest harmonic, $a_k$ is the amplitude of the $k^{\rm th}$ harmonic, $\omega_0$ is the fundamental frequency and $\phi_k$ is the absolute phase of the $k^{\rm th}$ harmoninc.

Then you can estimate $(a_k,\omega_0, \phi_k)$ by minimizing
$$
\left| x[n] - \sum_{k=K_1}^{K_2} a_k \sin(k\omega_0 n + \phi_k) \right|^2
$$
to find $(\hat{a}_k,\hat{\omega}_0, \hat{\phi}_k)$ and then look at the residual:
$$
\hat{x}_h[n] = x[n] - \sum_{k=K_1}^{K_2} \hat{a}_k \sin(k\hat{\omega}_0 n + \hat{\phi}_k)
$$

This should allow some cancellation of the harmonic background noise, but won't be real time.  There are algorithms that can do the estimation in real-time, but you're best trying this first on recorded data to see if it works.