Thank you for allowing me to assist you, my name is Joe and I will be happy to assist you with your concern today.
It really depends, what you could simply have is a bad motor mount or shaft coupler, bent driveshaft and or pump issue, with that said you could also have a serious problem internal be it top or bottom end. Pull a compression test and make sure that is ok, next check your motor mounts, one could simply be broken or multiple broken as well as loose exhaust, check driveshaft to be straight, if engine and pump as not aligned you will have a vibration to major noise depending on how bad.
If the problem is engine internal related then you can listen with a stethoscope to try and see where the sound is but a tear down will be required to fully diagnose at that point.
If it is as bad as you make it sound, it is not good and is serious enough to warrant not using it until it gets looked at, even if it is something small, if left unfixed it could lead to major problems.
Please rate my answer when ready. Bonuses are appreciated.
If for any reason you are not satisfied please reply back to me for further assistance to which I will be glad to help you. If you have any problems for any reason please reply back and let me know.
Thanks again for using justanswer and have a great day!
If you need any further assistance you can reply back at any time for my help.
I'd be happy to answer any other questions you have even if not related.