voice quivering on lower register

[vocal_help22]

hey, i am having an issue with my voice that i can't figure out. when i sing in a lower register i have a vibrato that sounds kind of like i'm straining and that i can't quite hit the note, and i don't know what is causing it. i'm trying to sing without any vibrato. it doesn't happen all the time. when it does happen, if i sing louder/softer it doesn't go away. sometimes, i can really nail these notes, but often they don't sound that strong. so i think maybe there's some inconsistency in my technique that i am not aware of / understanding.

i have been singing for about 10 years. i'm a tenor, i sing in a choir also. this warbling sound never arises when i sing higher. but on the lower notes, this warbling is a common feature.

i have uploaded a demo of a phrase where you can hear it - there are two files, one has a guitar backing track, the other is just my voice. here is the link:

https://soundcloud.com/analogue-social/sets/vocal-flutter?si=f1fbe974a76445e594e6a05fe9393953&utm_source=clipboard&utm_medium=text&utm_campaign=social_sharing

the words i am singing are: "along the burning grass, the deposit's a check"

and you can hear the flutter on the word 'grass'

other info: have done singing lessons for about 6 months to learn how to manage breath properly. also i have allergies / post-nasal drip that i manage with nasal irrigation, no steroid sprays tho.

does anyone know what might be causing this? breath / nasal drip / improper technique?

many thanks for any advice!!